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Abstract

The Neogene stratigraphic section in the Split Mountain area exposes megabreccia deposits up to 12 km long with volumes up to 3 × 108 m3. Shattered-rock domains still portray the bedrock distribution of lithologies. Jigsaw-puzzle fabric occurs on a variety of scales from microscopic to outcrop. Broken and stretched pegmatites tend to rise upward as step-ups in the inferred down-flow directions.

Upper Miocene subaerial megabreccias about 65 m thick disturbed the underlying strata to depths less than a meter during their emplacement. This includes producing grooved and decapitated stones both in the substrate and below shear surfaces especially within the basal few meters of the megabreccia deposits. The lower portions of a megabreccia are rich in step-ups, ramps, and crushed-rock streamers that rise upward in the down-flow direction.

After flooding of the basin by the ancestral Gulf of California, a lower Pliocene megabreccia moved across the sea floor deforming underlying sedimentary layers by injections and sunken megabreccia lobes that locally caused tightly folded bottom-sediment packages >35 m thick to rise as diapirs. Near the leading edge, on the southwest corner of the deposit, there is a small volume of more traditional sandy conglomerate deposited as the mass rapidly slowed and stopped.

Both subaerial and subaqueous megabreccias contain lithologic domains that preserve the distribution of bedrock lithologies, jigsaw-puzzle fabric, step-ups and crushed-rock streamers; these features all require non-turbulent flow. These huge volumes of shattered bedrock moved 10–12 km distance in late Miocene as dry subaerial masses, and again across the floor of an early Pliocene inland sea. All the observed features strongly indicate flow as sturzstroms.

Introduction

The Salton Trough region is the northern portion of the Gulf of California rift basin. It apparently began forming as a major half-graben during regional crustal extension that occurred in much of western North America during Miocene time (Frost et al., 1996). In the western Salton Trough, the early extension was accommodated on “… a linked system of stacked normal faults that predominantly tilt to the east.” (Frost et al., 1996, p. 81). Movement along these faults created a Miocene topography of high and isolated mountain blocks (e.g., Vallecito and Fish Creek) separated by basins. Sediments filling one of these basins, the Fish Creek-Vallecito basin, are the subject of this study (Fig. 1). The Fish Creek-Vallecito basin contains a Miocene to Pleistocene sedimentary section that is 5 km thick and records the rift history of the region (Kerr, 1984; Winker, 1987; Kerr and Kidwell, 1991; Winker and Kidwell, 1996). The lower part of the stratigraphic section is exposed exceptionally well in the walls of Split Mountain Gorge (Fig. 2).

Figure 1.

Location map of the Fish Creek-Vallecito basin near the northwestern margin of the Salton Trough.

Figure 1.

Location map of the Fish Creek-Vallecito basin near the northwestern margin of the Salton Trough.

Figure 2.

Index map for the northern part of the Fish Creek-Vallecito basin. The Anza-Borrego Desert State Park administers most of the area. Exceptional and readily accessible exposures are found in Split Mountain Gorge.

Figure 2.

Index map for the northern part of the Fish Creek-Vallecito basin. The Anza-Borrego Desert State Park administers most of the area. Exceptional and readily accessible exposures are found in Split Mountain Gorge.

The Fish Creek-Vallecito basin strata contain angular unconformities and differing dips indicating that faulting continued during deposition. Hence, the stratigraphic section provides a record of the progressive uplift and erosion of the basement rocks. The sedimentary rocks contain suites of conglomerate facies with varying clast compositions, sizes and shapes that reflect diverse source areas, different modes of transport and varying transport distances. Of special interest here are two upper Miocene megabreccias and one lower Pliocene megabreccia. The megabreccias were emplaced during catastrophic events as high-velocity flows. These flows are referred to as sturzstroms following the terminology introduced by Heim (1882, 1932) and repopularized by Hsü (1975).

Regional stratigraphic relationships

Neogene rocks exposed along the western margin of the Salton Trough have a long history of published and unpublished study. The seminal work of Dibblee (1954) provided a stratigraphic scheme for the Neogene rocks of the Imperial Valley. Woodard (1963, 1974) offered revisions to Dibblee's nomenclature.

Ruisaard (1979) and Kerr (1982) mapped (1:24000 scale) the area from the southern flanks of the Fish Creek Mountains through Split Mountain and to the eastern slopes of the Vallecito Mountains (Fig. 3). Their mapping focused on the lower Neogene continental sedimentary and volcanic rocks; attention was also paid to the overlying largely marine Neogene section. Kerr (1982, 1984) mapped sedimentary facies (Fig. 3), and recorded sedimentologic features, paleocurrent data and contact relationships. These efforts led to modifications of earlier stratigraphic schemes (Kerr, 1984; and Kerr's remarks in Kerr and Kidwell, 1991). The Fish Creek-Vallecito basin stratigraphy was also examined by Winker (1987; Winker and Kidwell, 1996) with detailed attention given to the upper part of the section.

Figure 3.

Geologic map of the area in and to the east of Split Mountain Gorge. Modified from Kerr (1982).

Figure 3.

Geologic map of the area in and to the east of Split Mountain Gorge. Modified from Kerr (1982).

As with most continental deposits, the age of the facies, which includes the two lower megabreccias, that make up the Anza Formation are not well constrained. The predominately basaltic volcanic rocks (formally known as the Alverson Andesite; Woodard, 1974) have yielded K-Ar radiometric ages ranging from 24.8 ± 7.4 to 14.9 ± 0.5 Ma. The braided fluvial and alluvial-fan facies of the Anza Formation are interstrati-fied with (e.g., Red Rock Canyon area, Fig. 2) and contain clasts derived from the volcanic rocks (see Kerr, 1984, for further discussion). Thus, the Neogene continental sedimentary rocks are, at least in part, time equivalent to the volcanic rocks (Fig. 4). The lower two megabreccias conformably rest on the Anza alluvial-fan facies in the vicinity of Split Mountain and on the braided-stream facies along the southern flanks of the Fish Creek Mountains (Fig. 3).

Figure 4.

Formal and informal stratigraphic nomenclature for the Neogene section along the western margin of the Saltan Trough.

Figure 4.

Formal and informal stratigraphic nomenclature for the Neogene section along the western margin of the Saltan Trough.

Age of the largely marine sedimentary rocks (the upper three divisions of the Split Mountain Formation of Woodard, 1974; Fig. 4) above the Anza Formation is established from the microfossil content and magnetic stratigraphy. Late Miocene and early Pliocene foraminifera, as well as other microfossils, have been collected from thin mudstones within and just above the Fish Creek Gypsum and graded beds exposed at the head of Split Mountain Gorge (Stump, 1972; Ingle, 1974; Dean, 1988; Winker and Kidwell, 1996). Johnson et al. (1983) determined a 4.3 Ma age, based on magnetic stratigraphy, on beds ∼200 m stratigraphically above the head of Split Mountain Gorge.

In Kerr's view (1982, 1984, in Kerr and Kidwell), an unconformity separates the lower to middle Miocene continental sedimentary and volcanic rocks from the overlying upper Miocene to lower Pliocene largely marine sedimentary rocks.

The ages of the three megabreccias (sturzstrom deposits) are established based on their position within the Neogene fill of the Fish Creek—Vallecito basin. The lower two megabreccias are middle to late Miocene in age depending on the magnitude of the superjacent unconformity. The upper megabreccia is early Pliocene in age. The sturzstrom events were geologically instantaneous, thus the megabreccias are marker beds within the basin fill.

Paleogeographic setting

In the Split Mountain region, Miocene andesitic volcanic rocks (22 to 14 Ma) and local continental sediments accumulated in an extended terrane. As extension evolved into rifting, larger volumes of sediments were deposited. In the Split Mountain area, a large alluvial fan built eastward from the Vallecito Mountains into the side of a major northward-flowing braided stream. Kerr's (1982, 1984) paleogeographic reconstruction for the early and middle Miocene showed that the Split Mountain area was the site of sedimentation in a half graben, the axis of which today trends north-northeasterly. The elevated areas of the Vallecito Mountains west of Split Mountain, and the Fish Creek Mountains lying to the east were likely to have been half-horst blocks. Coarse predominantly intermediate plutonic-basement detritus was derived from the uplifted Vallecito Mountains and deposited as debris-flow-dominated alluvial fans that prograded to the east-northeast (Kerr, 1982, 1984). The fans built into a basin-axis drainage characterized by deep braided-channel reaches draining northward with a conglomerate-clast assemblage that indicates a more diverse provenance.

The extension-created topography of highlands (Vallecito and Fish Creek Mountains) with the intervening Fish Creek-Vallecito sedimentary basin set the stage for voluminous sub-aerial sturzstrom events. Exposed in Split Mountain Gorge are the deposits of sturzstroms that flowed over the Miocene paleotopography. Conditions were prime for sturzstroms in late Miocene time. (1) The Vallecito Mountains stood high with a steep eastern face. (2) The plutonic rocks on top of the mountains were deeply fractured due to being the sole of detachment faulting. (3) The active tectonic processes in the region probably generated strong earthquakes that triggered large rockfalls that transformed into sturzstroms.

The stratigraphic section above the youngest subaerial sturzstrom records an abrupt change to a marine environment. Marine strata include the Fish Creek Gypsum, a relatively pure mass (>90%–95% CaSO4) resting on a basement-rock topography with considerable relief (Dean, 1988, 1996). The gypsum shows no cycles and no evidence of subaerial exposure, and it contains very little terrigenous sediment. Dean (1988) interpreted the origin of the Fish Creek Gypsum as precipitating from evaporating seawater, whereas Jefferson and Peterson (1998) interpret it as precipitating around submarine hydrothermal vents. To the west of Split Mountain Gorge, shallow-marine sandstone and mudstone undergo a lateral transition westward into alluvial-fan deposits.

The Fish Creek Gypsum also intertongues with alluvial-fan deposits north of Split Mountain Gorge. These alluvial-fan deposits have paleocurrent indicators and maximum clast-size variations indicating slopes away from the Vallecito Mountains and away from a source some 5 km north of the Gorge. It appears that the early to middle Miocene half-graben basin was undergoing significant tectonic reorganization in late Miocene and early Pliocene time. The paleogeographic setting was, from west to east, a Vallecito Mountains bedrock highland with a large eastward-prograding fan delta, a confined marine basin with gypsum and/or anhydrite precipitating, and a Fish Creek Mountains bedrock highlands. In early Pliocene time, a voluminous rockfall from the Fish Creek Mountains shattered and flowed south-southwest as a submarine sturzstrom.

MIOCENE SUBAERIAL STURZSTROM DEPOSITS

In Split Mountain Gorge, Woodard (1963, 1974) referred to the voluminous megabreccia as the lower fanglomerate member of the Split Mountain Formation. Robinson and Threet (1974) called it the lower breccia member and suggested it was deposited by a catastrophic sedimentary process probably involving an earthquake trigger and air-lubricated avalanching. Robinson and Threet felt that since the deposit thins to the west it must have come from the east in the Fish Creek Mountains.

Kerr (1982, 1984) described the mass as a sedimentary megabreccia, a landslide deposit derived from a western source in the Vallecito Mountains from whence it rapidly traveled as an air-cushioned landslide. Winker (1987) called the deposit the lower boulder bed and suggested the possibility that emplacement could have been under water, at least in part. Winker and Kidwell (1996) refer to it as the lower megabreccia, a non-marine landslide. Kerr (1984) noted a thick disturbed zone of alluvial-fan beds beneath the megabreccia (Fig. 5). This description was incorporated into a facies model for large rock avalanche deposits by Yarnold and Lombard (1989).

Figure 5.

View southeast of Miocene strata in 150 m high wall of Split Mountain Gorge. In lower left are alluvial-fan beds; in center is the red-and-gray sturzstrom deposit; at top, crossed by arrow, is vertical wall of Split Mountain megabreccia.

Figure 5.

View southeast of Miocene strata in 150 m high wall of Split Mountain Gorge. In lower left are alluvial-fan beds; in center is the red-and-gray sturzstrom deposit; at top, crossed by arrow, is vertical wall of Split Mountain megabreccia.

Kerr and Abbott (1996) measured a stratigraphic section up the slope shown in Figure 5 and did not find a disturbed zone of rumpled alluvial-fan beds beneath the massive cliff-forming megabreccia, but instead found two megabreccias. The apparent disturbed substrate of convoluted alluvial-fan beds in the middle portion of the Gorge wall is another shattered plutonic-rock breccia with jigsaw-puzzle fabric; it is the product of an earlier sturzstrom event. Closer inspection revealed sediment-filled stream channels cut in the lower sturzstrom deposit before the bigger one arrived (Fig. 6). Instead of causing a thick zone of disturbed and rumpled substrate beds, the younger sturzstrom flowed across the ground surface slicing through exposed boulders producing decapitated stones (Fig. 7).

Figure 6.

Closer view of contact between red and gray sturzstrom mass and overlying Split Mountain sturzstrom deposit. Arrow points to 3 m-thick. sediment-filled stream channel formed between the two sturzstrom events.

Figure 6.

Closer view of contact between red and gray sturzstrom mass and overlying Split Mountain sturzstrom deposit. Arrow points to 3 m-thick. sediment-filled stream channel formed between the two sturzstrom events.

Figure 7.

Closer view of Figure 6 stream-channel deposit. Arrow points to 0.45 m-diameter boulder sliced in half by Split Mountain sturzstrom leaving a grooved surface on a decapitated stone.

Figure 7.

Closer view of Figure 6 stream-channel deposit. Arrow points to 0.45 m-diameter boulder sliced in half by Split Mountain sturzstrom leaving a grooved surface on a decapitated stone.

Two subaerial sturzstroms flowed, in part, over the same geographic area, close together in geologic time. The earlier deposit is the red and gray sturzstrom and the superjacent deposit is the Split Mountain sturzstrom. Erosion following emplacement of the red and gray sturzstrom, plus burial by younger sediments, leave a restricted area of good outcrops, mainly in and near the west and east walls of Split Mountain Gorge. The Split Mountain sturzstrom crops out for seven kilometers and thus received more study.

Field relations of the Split Mountain megabreccia

In the east wall of Split Mountain Gorge, the Split Mountain sturzstrom deposit crops out as an abrupt 45 m high vertical face atop the red and gray sturzstrom deposit (Fig. 5). The Split Mountain sturzstrom deposit is an olive gray (5Y 6/1), extremely poorly sorted, well-indurated, matrix-supported megabreccia. The deposit has an overall reverse grading with the majority of the megaboulders riding on top. It has a sharp basal contact across the red and gray sturzstrom deposit. The upper surface is hummocky and is mostly overlain by Fish Creek Gypsum (Fig. 8) but toward the southwest it is overlain by greenish fan-delta mudstone and sandstone (Fig. 9).

Figure 8.

Close-up view of Fish Creek Gypsum precipitated over boulders on top of Split Mountain sturzstrom deposit. Arrow points to coarse sandstone from subaerial fallout. Straw hat for scale.

Figure 8.

Close-up view of Fish Creek Gypsum precipitated over boulders on top of Split Mountain sturzstrom deposit. Arrow points to coarse sandstone from subaerial fallout. Straw hat for scale.

Figure 9.

View east up southwest-dipping stratigraphic surface on top of Split Mountain sturzstrom megabreccia. Note concentration of tonalite megaboulders on top of Split Mountain sturzstrom. On skyline to left is light-colored Fish Creek Gypsum deposited on Split Mountain sturzstrom. On right are marine fan-delta mud-stone and sandstone beds deposited on top of Split Mountain sturzstrom deposit. On upper right skyline atop the 240 m high slope is Pliocene submarine megabreccia.

Figure 9.

View east up southwest-dipping stratigraphic surface on top of Split Mountain sturzstrom megabreccia. Note concentration of tonalite megaboulders on top of Split Mountain sturzstrom. On skyline to left is light-colored Fish Creek Gypsum deposited on Split Mountain sturzstrom. On right are marine fan-delta mud-stone and sandstone beds deposited on top of Split Mountain sturzstrom deposit. On upper right skyline atop the 240 m high slope is Pliocene submarine megabreccia.

The Split Mountain sturzstrom deposit extends from the western wall of Split Mountain Gorge 5.4 km southeast to the mixed plutonic and metamorphic massif of the Fish Creek Mountains where it takes a right-angle turn for 1.6 km to the northeast (Fig. 3). Calculating the volume of the Split Mountain megabreccia involves compensating for erosion on part of the north side and burial on part of the south side. A rough estimate of the Split Mountain sturzstrom volume is 3 × 108 m3.

In later pages it is demonstrated that the Split Mountain sturzstrom came from the Vallecito Mountains. If this is the case, the Split Mountain sturzstrom traveled 5.2 km from the Vallecito Mountains front before leaving deposits found today. Then the Split Mountain sturzstrom outcrop runs 5.4 km southeast before a 1.6 km dogleg to the northeast, resulting in a total runout of 12.2 km.

Subjacent rocks.

The Split Mountain megabreccia rests on the red and gray megabreccia in Split Mountain Gorge. The red and gray megabreccia extends from ∼0.2 km west of the Gorge west wall to 0.6 km east of the Gorge. The eastern extent is uncertain because the contact is covered with modern debris eroded from the overlying Split Mountain megabreccia. The red and gray megabreccia is ∼65 m thick and lies on Anza Formation alluvial-fan strata.

Southeast of Split Mountain Gorge, the Split Mountain megabreccia rests on upper to middle alluvial-fan deposits (up to 12 m thick debris-flow beds interstratified with sheetflood sandstones). Continuing to the southeast, the megabreccia rests on lower alluvial fan deposits (thin mudflow deposits and sheetflood sandstones), and finally the megabreccia rests directly on braided-stream deposits of the axial drainage system (paleocurrent indicators to the north). Farther southeast. the Split Mountain megabreccia bends around the basement rock of the Fish Creek Mountains (Fig. 3).

Superjacent rocks.

The subaerial Split Mountain sturzstrom deposit is overlain to the east and south by a thick blanket of Fish Creek Gypsum (Fig. 8) and to the southwest by fan-delta mudstone and sandstone beds (Fig. 9). The Split Mountain sturzstrom is a significant marker bed separating subjacent nonmarine strata from superjacent predominantly marine beds. Below the Split Mountain sturzstrom lie a 1.5 km thickness of braided-stream and alluvial-fan strata; above the Split Mountain sturzstrom is a 4+ km thick section of fan-delta, tidal-flat, and Colorado River delta and delta-plain strata.

Paleogeographic implications

Before the Split Mountain sturzstrom event, the Vallecito Mountains were shedding coarse sediments onto an eastward-prograding large alluvial fan that built into the side of a valley containing a major northward-flowing braided stream. Then, possibly initiated by one of the great earthquakes that had to occur in this extending region, a 3 × 108 m3 mass of pegmatite-laced tonalite fell, shattered, and flowed eastward down the alluvial fan, over small tributary streams, into and up the large braided stream valley including flowing around a salient of the Fish Creek Mountains (Fig. 10). The paleotopography confined the flowing mass creating a linear body of megabreccia that came to rest on the lower parts of the alluvial-fan system and within the axial-valley of the braided stream. The megabreccia rests solely on top of nonmarine sedimentary rocks.

Figure 10.

Schematic paleogeographic map of the Split Mountain area in latest Miocene time. The Split Mountain sturzstrom began as a plutonic-rock fall on the steep face of the Vallecito Mountains, which shattered and flowed eastward down a large alluvial fan and then up a braided-stream valley around the Fish Creek Mountains.

Figure 10.

Schematic paleogeographic map of the Split Mountain area in latest Miocene time. The Split Mountain sturzstrom began as a plutonic-rock fall on the steep face of the Vallecito Mountains, which shattered and flowed eastward down a large alluvial fan and then up a braided-stream valley around the Fish Creek Mountains.

Ocean water flooded the area soon after the Split Mountain sturzstrom. The Split Mountain sturzstrom deposit is buried beneath both gypsum and anhydrite precipitated from sea water and a fan delta that prograded into the sea.

Lithologic characteristics

Domains.

The Split Mountain sturzstrom deposit is clearly separable into domains of different lithologic composition. In the walls of Split Mountain Gorge, the Split Mountain sturzstrom rock mass is almost totally composed of a foliated, moderately deformed, moderately altered tonalite. Quartz crystals have strong undulatory extinction. Subhedral clinopyroxene has thin rims of green amphibole. Brown biotite is common and has some secondary sphene. Secondary epidote and chlorite are present. The rock is cut by pegmatite dikes. The overall aspect of the tonalite is similar to the Jurassic batholith that formed before, and was altered by, the voluminous Cretaceous plutonism of the Peninsular Ranges batholith (M. Walawender, personal commun., 1996).

Toward the east, the Split Mountain sturzstrom deposit becomes much lighter in color reflecting a granodiorite source rock with pegmatite. Near the toe, there is a domain of granite with large quartz phenocrysts (Borron, 1999).

The Split Mountain sturzstrom deposit is composed of relatively pure masses of shattered bedrock that occupy different areas and show essentially no mixing. These spatial segregations almost certainly reflect their different bedrock positions before the sturzstrom event was initiated. This preservation of bedrock geologic relationships and stratigraphy in sturzstrom deposits has been noted by many authors beginning with Albert Heim (1882).

Texture.

The Split Mountain sturzstrom deposit is dominated by sand-, silt-, and clay-sized grains lacking a readily recognizable distribution grading of grain sizes. Gravel-sized clasts in the lower half of the deposit also do not show a distinct grading (Fig. 11). However, a dominant inverse grading exists in the coarse tail of the grain population, especially with the large boulder blocks that are concentrated near and at the top of the deposit along its entire length, but are absent in the lower and middle portions (Figs. 5, 9). These megaclasts are blocks of plutonic rock, some exceeding 40 m diameter. To estimate the abundance of megaclasts at the top of the Split Mountain sturzstrom deposit, an aerial photograph of the proximal deposit was scanned at 400% and 72 dpi (Borron, 1999). An area of about 4 km2 is shown in Figure 12A, and again after digital enhancement using a “finding the edges” algorithm (Fig. 12B). There are 198 megaclasts (10–44 m diameter) exposed in this 4 km2 area on top of the deposit.

Figure 11.

Macroscopic grain-size trends in a vertical section through the Split Mountain sturzstrom deposit. Analysis made on photographs from stratigraphic section near peak 1704 (Fig. 2).

Figure 11.

Macroscopic grain-size trends in a vertical section through the Split Mountain sturzstrom deposit. Analysis made on photographs from stratigraphic section near peak 1704 (Fig. 2).

Figure 12.

A. Aerial photograph of a 4 km2 area of the upper surface of the Split Mountain Sturzstrom deposit; the Fish Creek Wash is to the left. B. The same image sharpened with the Finding-the-Edges filter to emphasize the megaclasts (>10 m diameter) exposed at the surface: there are ∼49 megaclasts per km2.

Figure 12.

A. Aerial photograph of a 4 km2 area of the upper surface of the Split Mountain Sturzstrom deposit; the Fish Creek Wash is to the left. B. The same image sharpened with the Finding-the-Edges filter to emphasize the megaclasts (>10 m diameter) exposed at the surface: there are ∼49 megaclasts per km2.

The vertical distribution of sediment sizes in the Split Mountain sturzstrom deposit seems to be the result of a ball-mill effect. Shattered plutonic-rock debris in the lower parts of the flow was smashed into ever-finer sediment by repeated collisions, while large blocks riding on top survived uncrushed. However, even the megaclasts on top commonly show impact-point scars, plus long and deep fractures testifying to violent collisions.

Subaerial fallout cap.

The upper surface of the Split Mountain sturzstrom deposit is covered with a discontinuous layer, up to 15 cm thick, of thinly bedded, medium to very coarse-grained sandstone composed of plutonic-rock detritus (Fig. 8). When the sturzstrom stopped moving, its deposit stood topographically high, thus it is unlikely that the sand was washed onto the top of the deposit by later water flows. The coarse sand must have settled from the airborne suspension, or dust cloud, created by the rock-fall impact that initiated the sturzstrom (Borron, 1999). It is likely that additional airborne sediment was formed by collisions occurring during flow.

A recent example of this process occurred in Yosemite National Park in California (Wieczorek et al., 2000). On July 10, 1996, about 3 × 104 m3 of tonalite and granodiorite fell from a steep cliff in two events only seconds apart. Upon impact, fragmented and pulverized rock formed a dense sandy cloud that flowed away from the impact point, depositing a cover of medium- to fine-grained sand as it flowed. For scale, the Yosemite rock fall and flow was ∼0.1% of the volume of the Split Mountain sturzstrom.

Fabric.

Grains at various size scales can be visually refitted to reconstruct what formally were much larger bodies. This is known as jigsaw-puzzle fabric. This fabric is best seen where meter- to several meter-diameter blocks have been shattered into innumerable small pieces, with very little mixing of the pieces. For example, white pegmatite veins in large blocks of gray tonalite have been broken into small pieces, yet not widely separated or mixed with matrix (Fig. 13). Even in microscopic views, individual grains of matrix can be refitted like pieces of a jigsaw puzzle.

Figure 13.

Canyon-wall exposure ∼4 m high inside the Split Mountain sturzstrom deposit. Entire photo is biotite tonalite with one white pegmatite indicated by black arrows. In upper right is a tonalite boulder with the white pegmatite. The remainder of the tonalite, and the same white pegmatite, are shattered and display jigsaw-puzzle fabric.

Figure 13.

Canyon-wall exposure ∼4 m high inside the Split Mountain sturzstrom deposit. Entire photo is biotite tonalite with one white pegmatite indicated by black arrows. In upper right is a tonalite boulder with the white pegmatite. The remainder of the tonalite, and the same white pegmatite, are shattered and display jigsaw-puzzle fabric.

The sturzstrom mass is shattered in situ and has negligible dispersion of broken clasts indicating that very little internal mixing took place during transport. The megaclasts at the top of the deposit must have started on top of the fallen bedrock mass, rode on the top of the flow, and stopped while still on top. The concentration of finer debris near the base of the Split Mountain sturzstrom deposit probably reflects the greater amount of shear stress imparted in the lower part of the fast-flowing sturzstrom.

Geochemistry.

Lithologic domains and jigsaw-puzzle fabric exist with obvious integrity at outcrop scale. But how closely does a large boulder resemble its surrounding fine sedimentary matrix (mud- and sand-sized grains) when analyzed geochemically? Has much mixing of shattered material occurred during transport? To gain insight into these questions, samples were taken from plutonic boulders in both the red and gray and Split Mountain sturzstrom deposits. In addition, matrix samples were collected about one meter away from each boulder. The samples were sent to the GeoAnalytical Laboratory at Washington State University and analyzed for 43 elements by X-ray fluorescence (XRF) and inductively coupled plasma-mass spectrometry (ICP-MS) techniques. The plots of clast versus nearby matrix chemistry are shown in Figure 14. The basic similarities of the clast and matrix analyses testify to the paucity of mixing that occurred during transportation.

Figure 14.

Chemical compositions of a large tonalite clast and the nearby matrix in the Split Mountain sturzstrom deposit.

Figure 14.

Chemical compositions of a large tonalite clast and the nearby matrix in the Split Mountain sturzstrom deposit.

Geochemistry also was employed to evaluate the differences in clay-size fractions of the red and gray and Split Mountain megabreccias versus the clay-rich mudstone beds of the subjacent braided-stream overbank and the superjacent locally derived fan-delta and the regionally derived muds delivered by the ancestral Colorado River (Table 1). Obvious differences appear. The compositions of the water-laid fine sediments are much lower in SiO2 and Na2O but are significantly higher in Al2O3, TiO2, FeO (total iron), MgO, K2O, P2O5. The chemical compositions of the fine sediments in the sturzstrom deposits do not resemble those of the water-laid deposits, but they are similar to the chemical compositions of representative tonalites from the Peninsular Ranges batholith (Table 1). The chemical analyses strongly suggest that the clay-size fraction of the sturzstrom deposits was created simply by shattering plutonic basement rock.

Table 1.

Matrix Geochemical Raw Data

SampleSiO2Al2O3TiO2FeOMnOCaOMgOK2ONa2OP2O5
Imperial Formation60.0418.400.745.160.040.733.033.821.900.17
    yellow mud
Fan-delta green55.7817.011.118.650.112.304.413.862.570.22
    mud
Split Mountain67.4015.870.563.400.063.571.823.143.440.12
    sturz. Matrix
Red and Grey67.8515.850.563.140.053.371.523.633.330.12
    sturz. Matrix
Braided stream56.1117.071.167.650.123.673.813.981.740.30
    red mud
Long Potrero67.3615.660.503.740.074.321.671.823.800.10
    tonalite
Morena Reservoir67.4015.540.554.180.104.081.091.264.740.15
    tonalite
SampleSiO2Al2O3TiO2FeOMnOCaOMgOK2ONa2OP2O5
Imperial Formation60.0418.400.745.160.040.733.033.821.900.17
    yellow mud
Fan-delta green55.7817.011.118.650.112.304.413.862.570.22
    mud
Split Mountain67.4015.870.563.400.063.571.823.143.440.12
    sturz. Matrix
Red and Grey67.8515.850.563.140.053.371.523.633.330.12
    sturz. Matrix
Braided stream56.1117.071.167.650.123.673.813.981.740.30
    red mud
Long Potrero67.3615.660.503.740.074.321.671.823.800.10
    tonalite
Morena Reservoir67.4015.540.554.180.104.081.091.264.740.15
    tonalite

X-ray Diffractometry.

Bulk-rock chemistry shows clear and definable differences between the fine sediments of water-laid versus sturzstrom deposits, but how similar are the mincralogical compositions? X-ray diffraction scans were run on fractions of the same samples analyzed chemically and reported in Table 1. The diffractograms for the water-laid samples from braided stream, fan delta and ancestral Colorado River delta, all have intense peaks for smectite, minor peaks for illite, and even lesser peaks for kaolinite. However, the sturzstrom-matrix samples have no peaks for phyllosilicate minerals (except for biotite); their peaks are for quartz, feldspar and other plutonic-rock minerals. Scans demonstrate that the sturzstrom deposit matrices never went through chemical weathering processes prior to transportation and that their histories are different from the local stream and fan delta, and extra-regional Colorado River-supplied sediments. The sediment populations in the sturzstrom masses formed by physical disintegration of pegmatite dike-bearing plutonic rocks, i.e., the shattering of a gigantic mass into innumerable angular fragments within the geologically trivial time span of about five minutes.

Paleoflow indicators and direction of transport for the Split Mountain sturzstrom

Numerous features within the sturzstrom deposit serve as paleoflow indicators. Eastward paleoflow directions were indicated by the following items.

Grooves and Striations.

The base of the Split Mountain sturzstrom deposit is sharp (Fig. 7). As the sturzstrom flowed across the alluvial-fan surface it sliced through some of the hard plutonic-rock boulders protruding above the surface. The term decapitated stones is herein introduced for this feature.

The planar surfaces of the decapitated stones have grooves and striations scored onto them. Additionally, some large boulders (e.g., 2.5 m diameter) lying on the ground were not decapitated but were overrun by the sturzstrom leaving grooves and striations that record its passage (Fig. 15). The grooves are subparallel, smoother in the direction of flow, and number as high as 25 on one rock surface. Measurements of groove directions at 44 sites ranged between 70° to 100° with a mean trend of 91.5°, uncorrected for any later tectonic rotations that may have occurred (Borron, 1999).

Figure 15.

A boulder protruding above the Miocene ground surface was overrun by the Split Mountain sturzstrom leaving a polished and grooved surface. Grooves are smooth toward the viewer and document a flow direction of 92°. The coin is 2.4 cm diameter.

Figure 15.

A boulder protruding above the Miocene ground surface was overrun by the Split Mountain sturzstrom leaving a polished and grooved surface. Grooves are smooth toward the viewer and document a flow direction of 92°. The coin is 2.4 cm diameter.

Step-ups and Ramps.

The pegmatite veins within tonalite boulders of the Split Mountain sturzstrom deposit are linear and randomly oriented, but where crushed, they commonly have a preferential orientation. The shattered pegmatites display a step-up geometry wherein their western ends are lower in elevation than their eastern ends, which are stepped-up higher (Fig. 16). The step-ups occur in the downflow direction and are attributed to the velocity gradient within a mass flow, with slower velocities at the base, and increasing velocity upward. Azimuth measurements of 48 step-ups in shattered pegmatites yield directions between 90° and 160° with a mean trend of 120°. This mean direction is not as accurate as the 91.5° mean trend of the grooves because the step-ups are three-dimensional features exposed on essentially two-dimensional canyon walls.

Figure 16.

Canyon-wall exposure cut through lower Split Mountain sturzstrom deposit. Shattered pegmatites step-up to the east (mimicked by black arrows), trending 105°. Staff is painted in decimeters.

Figure 16.

Canyon-wall exposure cut through lower Split Mountain sturzstrom deposit. Shattered pegmatites step-up to the east (mimicked by black arrows), trending 105°. Staff is painted in decimeters.

The basal Split Mountain sturzstrom deposit shows shear surfaces, or ramps, that step up eastward (Fig. 17). The ramps are seen in the walls of canyons incised subparallel to the paleoflow direction. It seems that as some basal Split Mountain sturzstrom debris stopped flowing, it was overrun by moving debris from behind.

Figure 17.

Ramp within the lower Split Mountain sturzstrom deposit in distal reaches. Split Mountain sturzstrom debris on left stopped and was overrun by debris on right. Ramp rises to the east in the paleoflow direction.

Figure 17.

Ramp within the lower Split Mountain sturzstrom deposit in distal reaches. Split Mountain sturzstrom debris on left stopped and was overrun by debris on right. Ramp rises to the east in the paleoflow direction.

Crushed-Rock Streamers.

Locally within the Split Mountain sturzstrom megabreccia there occur crudely conical-shaped crushed pegmatite masses herein referred to as crushed-rock streamers. The streamers taper in eastward and upward directions from the pegmatite block. It appears that after some blocks shattered, they were deformed within the flowing sturzstrom. Some crushed rock streamers are reminiscent of comets moving away from the Sun, i.e., the cores of the blocks lie to the west while tails of finer debris stream off to the east.

PLIOCENE SUBMARINE STURZSTROM DEPOSIT

A younger megabreccia is exposed at the head of Split Mountain Gorge. It has been called the upper fanglomerate by Woodard (1974), the landslide facies by Kerr (1982), the upper boulder bed by Winker (1987), and the upper breccia by Dean (1988). It is referred to here as the Fish Creek sturzstrom (Rightmer and Abbott, 1996). The Fish Creek sturzstrom deposit is resistant to erosion and holds up prominent southwest-facing dip slopes in much of the Split Mountain area. The deposit is essentially an unsorted breccia with dark grayish-olive matrix and clasts that vary in composition and size. Clasts range from less than a centimeter to in excess of 25 meters. The largest clasts are at the top of the Fish Creek sturzstrom deposit and some are represented by single or double concentric-closed contours on 1:24 000 U.S. Geological Survey topographic maps with 40 ft contour intervals.

Field relations of the Fish Creek megabreccia

The Fish Creek megabreccia outcrop has an elongated shape oriented northwest-southeast. Along the eastern margin of the outcrop, the Fish Creek sturzstrom deposit lies barely above the Fish Creek Gypsum (Fig. 3). In the central region, it typically occurs sandwiched within prodelta mudstone and sandstone. To the northwest, the megabreccia overlies coarser sediments from mudstone to sandstone into boulder-bearing sandstone beds (Fig. 18) and into sandy conglomerate beds. The increasing grain size of bottom sediments reflects travel across distal to medial fan-delta regions. Stratigraphic context indicates that the Fish Creek sturzstrom encountered seawater near its source area and traveled below sea level.

Figure 18.

Fish Creek sturzstrom megabreccia on top of fan-delta boulder-bearing sandstone beds at head of Split Mountain Gorge in 20 m high cliff.

Figure 18.

Fish Creek sturzstrom megabreccia on top of fan-delta boulder-bearing sandstone beds at head of Split Mountain Gorge in 20 m high cliff.

The varying grain sizes of the substrate sediments along the flow path had a significant effect upon the morphology of the basal portions of the Fish Creek sturzstrom deposit. Based upon morphological differences, four zones are defined (Fig. 19).

Figure 19.

Sketch of changes in Fish Creek sturzstrom morphology and bottom deformation from southeast (zone 1) to northwest (zone 4).

Figure 19.

Sketch of changes in Fish Creek sturzstrom morphology and bottom deformation from southeast (zone 1) to northwest (zone 4).

In zone 1, in the southeast, the sturzstrom mass exhibits a high level of coherence and there is little contamination of the mass via incorporation of bottom sediments. Breccia and substrata folds are large and simple.

Just east of Split Mountain Gorge (e.g., Crazycline Canyon, Fig. 2) are zone 2 outcrops showing the results of the Fish Creek sturzstrom reaching deeper marine water and abundant clay-rich beds in the substrate. The denser breccia mass injected and settled into the bottom muds. The bottom sediments folded and rose diapirically producing spectacular outcrop-scale convolute patterns including semivertical contacts exceeding 35 m in height (Fig. 20).

Figure 20.

Wall of Crazycline Canyon showing injected and sunk Fish Creek sturzstrom (FCS) breccia along each side of a large diapir of prodelta mudstone and sandstone. Outcrop is 35 m high.

Figure 20.

Wall of Crazycline Canyon showing injected and sunk Fish Creek sturzstrom (FCS) breccia along each side of a large diapir of prodelta mudstone and sandstone. Outcrop is 35 m high.

Common in zone 3 are large injections of Fish Creek sturzstrom into sandy bottom sediments. Deformation is local and the overall stratigraphy is preserved. Although tongues of Fish Creek sturzstrom broke loose and descended into the seabed the main Fish Creek sturzstrom mass continued to travel along the water-sediment interface apparently unimpeded.

Northwest of Lycium Wash (Fig. 2) are zone 4 outcrops where contamination of the Fish Creek sturzstrom appears to have become complete. The three-dimensional jigsaw-puzzle fabric and lithologically similar domains are not present; the clasts are dispersed throughout the matrix. There are no folds or diapiric structures discernible. In addition, a change in flow dynamics is indicated by the absence of the shattered and stretched-out pegmatite dikes observed at virtually every sturzstrom outcrop. Fragmented dike material is here dispersed and deposited along with other rock types in a random fabric within a series of bedded conglomerates. The beds reveal a stacked sequence of matrix-dominated sturzstrom-suite breccias with the largest clast being ∼2 m diameter. Beds are from 1 to 3 meters thick, laterally persistent, and internally chaotic.

The areal extent of the Fish Creek sturzstrom has been defined by mapping (e.g., Kerr, 1982; Winker, 1987). Calculation of Fish Creek sturzstrom volume is complicated by several factors: (1) the deposit has been tilted and eroded, (2) it is offset by numerous faults, (3) the interaction of the flowing sturzstrom with the bottom sediments resulted in large injections of sturzstrom and upward-rising diapirs of bottom sediments, and (4) the upper surface has depositional relief. Using only exposed outcrop in the calculations, a probable volume based upon a length of 11 km, average width of 1.6 km and an average thickness of 17 m is 3 × 108 m3. This deposit is about the size of the well-known Blackhawk slide (Shreve, 1968; Johnson, 1978).

Lithologic characteristics

Domains.

The megabreccia outcrop extends 11 km between the Fish Creek Mountains and the foot of the Vallecito Mountains and displays numerous lithologically similar domains with abrupt boundaries. Typically a domain consists of pervasively fractured clasts of a given composition that arc shattered and separated by an intervening matrix composed primarily of comminuted fragments of the same composition. It seems likely that they were once part of a larger block that broke up during transport. Outcrop inspection of the breccia sheet reveals that lithological domains are scale independent from cm to km (Fig. 21).

Figure 21.

Jigsaw-puzzle fabric within pegmatite and gneiss megaclasts at a Fish Creek sturzstrom domain boundary. Note the lack of mixing. Scale bar is in centimeters.

Figure 21.

Jigsaw-puzzle fabric within pegmatite and gneiss megaclasts at a Fish Creek sturzstrom domain boundary. Note the lack of mixing. Scale bar is in centimeters.

The composition of the Fish Creek sturzstrom is polymictic containing pegmatitic granodiorite and tonalite, leucocratic granite, pegmatite, mica schist plus other metamorphic rocks including biotite gneiss and marble.

Fabric.

From sand-size grains to 30 m diameter boulders, most clasts within the Fish Creek megabreccia are pervasively fractured. Most clasts, regardless of composition, contain some pegmatite dikes, frequently up to 35% by volume. Although thoroughly fractured, the pegmatite dikes remain easily recognizable in the matrix (Fig. 22). The geometric relationship of the fragments commonly allows the observer to visually put the pieces back together. All major rock types in the sturzstrom breccia suite, regardless of their size, commonly exhibit three-dimensional jigsaw-puzzle fabric.

Figure 22.

Cliff wall at head of Split Mountain Gorge shows Fish Creek sturzstrom deposit on skyline with characteristic shattered, but not dispersed, white pegmatites.

Figure 22.

Cliff wall at head of Split Mountain Gorge shows Fish Creek sturzstrom deposit on skyline with characteristic shattered, but not dispersed, white pegmatites.

The preservation of jigsaw-puzzle fabric in the Fish Creek sturzstrom after traveling several km from its source, and mostly subaqueously, indicates that large-scale mixing or overturning could have not been taking place during transport.

Thin-Section Petrography.

Thin sections of Fish Creek sturzstrom matrix collected in plutonic-rock domains show quartz, feldspar, lithic fragment (QFL) composition percentages ranging from Q76F24L0 to Q49F51L0 and from Q71F10L19 to Q46F24L30 with all lithic fragments being plutonic. The plutonic rocks are quartz rich and the feldspar assemblage has abundant potassium feldspar, and microcline and myrmekite are common.

Geochemistry.

Geochemistry was employed to evaluate the amount of mixing that occurred during transport. Six samples were collected from Fish Creek sturzstrom clasts (plutonic, schist, pegmatite) and surrounding matrix within one meter of each clast. The analytical data from each sample were normalized to aluminum. The log-transformed chemical data contrasting a plutonic clast to its nearby matrix plot near unity (Fig. 23). The results indicate that plutonic clasts and their surrounding matrix have similar chemical compositions and thus underwent very little mixing during transport. The Fish Creek sturzstrom matrix composition is that of unweathered plutonic bedrock, and it is unlike that of marine sedimentary-rock layers above and below it.

Figure 23.

Geochemical similarities between Fish Creek Sturzstrom bedrock clasts (plutonic, schist, pegmatite) and shattered matrix about one meter away from each respective clast. A scarcity of mixing is shown on a geochemical basis.

Figure 23.

Geochemical similarities between Fish Creek Sturzstrom bedrock clasts (plutonic, schist, pegmatite) and shattered matrix about one meter away from each respective clast. A scarcity of mixing is shown on a geochemical basis.

The geochemical data obtained from a large schist clast and its nearby matrix have some element-ratio patterns that diverge from unity showing some compositional differences between clast and matrix (Fig. 23). The pattern suggests a modest chemical affinity in that most ratios plot near unity. Geochemical comparisons were made between Fish Creek sturzstrom mica-schist clasts and the Julian Schist on top of the Peninsular Ranges batholith. Normalized geochemical data from mica-schist clasts were matched with the Julian Schist average composition of Murray (1995). The element ratios, except for CaO and MnO, plot near unity and suggest a reasonable chemical similarity.

Similar geochemical analyses were done on a pegmatite dike clast and its nearby matrix (Fig. 23). Most element ratio patterns separate from unity indicating that compositional differences exist for most elements. These differences may be due to the matrix containing shattered host rock as well as pegmatite.

The chemical similarities of megabreccia matrix and nearby clasts provide evidence of the relative lack of mixing of debris during transportation.

Paleoflow indicators and direction of transport

Fold axes.

Outcrop-scale folds, including axial orientation and vergence, can be credible kinematic indicators. In many landslides early fold patterns are formed due to a shear couple at the base of the landslide. Interference patterns are produced when early folds are refolded by slip transverse to the early fold axes (Hanson, 1971). Refolding creates vergence directions (dextral or sinistral) for folds. A line separating dextral from sinistral vergences has been shown by Hanson (1971) to be the slip line (direction of transport).

Winker and Kidwell (1996, p. 322) measured and plotted folds in the sediments below the Fish Creek sturzstrom and came up with a pattern. They state that: “Fold axes show a preferred NE-SW alignment, but no preferred direction of overturning. We consequently think the fold axes may be aligned with the transport direction of the megabreccia rather than transverse to it as commonly supposed.”

Isopach analysis.

The Fish Creek sturzstrom deposit was mapped using a laser-range finder and Global Positioning System in the field with the data placed into Arcview (Washburn, 2001).

The resulting isopach map shows the Fish Creek sturzstrom deposit with a thicker southern edge, which probably represents the leading edge of the flow. This analysis suggests that the Fish Creek sturzstrom originated in the northern Fish Creek Mountains and then flowed south-southwest.

Source area of the Fish Creek sturzstrom

Robinson and Threet (1974) stated that the breccia deposit thins to the east and proposed an origin as an air-lubricated avalanche from a western source in the ancestral Vallecito Mountains. Pappajohn (1980) favored a north or northeast source based upon the appearance of marble clasts in the breccia. Rightmer and Abbott (1996) suggested paleoflow was westward from a Fish Creek Mountains source toward the Vallecito Mountains. Winker (1987) measured axial orientations of disharmonic folds in the sandstone below the Fish Creek sturzstrom and suggested a preference for northeast-southwest orientations, but the data did not present a preferred sense of vergence. Then in 1996, Winker and Kidwell stated that the Fish Creek sturzstrom source lay “in now-buried basement terrain to the south and southwest” and that flow was to the northeast. Thus prior workers have suggested that the Fish Creek sturzstrom source lay to the west, the south, the east, and the north.

Lithologic Correlation.

The Fish Creek Mountains lying northeast, east and southeast of Split Mountain contain major outcrops of marble and other metasedimentary rocks (Morton, 1977). The almost inaccessible Waters deposit contains a 900 m thick section of marble (Paleozoic?) interbedded with mica schist, quartzite and gneiss (Morton, 1977). Field reconnaissance has not revealed marble outcrops in the Vallecito Mountains nor in the stream channels draining them to the south. Both the Vallecito and Fish Creek Mountains contain granitic rocks but their overall appearances are different. Reconnaissance mapping suggests that the Vallecito Mountains contain lesser amounts of dikes (<10%), whereas the northern Fish Creek Mountains have voluminous dikes (>35%). Gravel counts in south-flowing stream channels draining the Vallecito Mountains show less than 1% contain dike material and that metamorphic clasts are less than 1% of total clasts. In north-flowing streams draining the Fish Creek Mountains, 35% of the clasts contain some dike rock and metamorphic clasts are greater than 20% of the total clast population. In short, basement rocks in the Fish Creek Mountains closely resemble the Fish Creek sturzstrom clast suite suggesting that the Fish Creek Mountains are the source area of the sturzstrom, hence its name—Fish Creek sturzstrom.

COMPARISON OF THE SUBAERIAL AND SUBMARINE STURZSTROM DEPOSITS

The Neogene sedimentary section in the Split Mountain area contains world-class exposures of sediments left by late Miocene subaerial and early Pliocene submarine sturzstrom events. The late Miocene sturzstroms flowed eastward from the Vallecito Mountains over alluvial fan and braided stream topography never encountering standing water. The early Pliocene sturzstrom apparently flowed southwest from the Fish Creek Mountains and most of its path was through the sea. How different are the deposits resulting from subaerial versus submarine flow? Surprisingly, their similarities are much greater than their differences (Table 2). The sturzstrom deposits share the same characteristics within their masses, but differ along their bottom contacts. Subaerial sturzstroms decapitate and groove boulders sitting on the topography but do not significantly disturb the substrate. The submarine sturzstrom deeply disturbed its substrate via injections, sinking, and diapirs of sea-bottom sediments.

Table 2.

Characteristics of Subaerial and Submarine Sturzstrom Deposits

FeatureSplit Mountain sturzstromFish Creek sturzstrom
Approx. volume300 × 106m3300 × 106m3
Lithologic domainsPervasivePervasive
Mixing w/in sturzstromNoNo
Preservation of bedrock relationsYesYes
Jigsaw-puzzle fabricAbundantAbundant
Shattered pegmatitesAbundantAbundant
Crushed-rock streamersCommonCommon
Decapitated stonesYesNot seen
Grooved & striated clastsYes, near baseNot seen
Shear surfaces within basal few mYesNot seen
Depth of substrate disturbance1 m>35 m
Folds in subjacent strataNoYes
Sandstone dikesYes, rareYes, esp. distally
Diapirs intruding sturzstromNoYes
Injected lobes of sturzstromNoYes
Megaclasts concentrated up topYesYes
FeatureSplit Mountain sturzstromFish Creek sturzstrom
Approx. volume300 × 106m3300 × 106m3
Lithologic domainsPervasivePervasive
Mixing w/in sturzstromNoNo
Preservation of bedrock relationsYesYes
Jigsaw-puzzle fabricAbundantAbundant
Shattered pegmatitesAbundantAbundant
Crushed-rock streamersCommonCommon
Decapitated stonesYesNot seen
Grooved & striated clastsYes, near baseNot seen
Shear surfaces within basal few mYesNot seen
Depth of substrate disturbance1 m>35 m
Folds in subjacent strataNoYes
Sandstone dikesYes, rareYes, esp. distally
Diapirs intruding sturzstromNoYes
Injected lobes of sturzstromNoYes
Megaclasts concentrated up topYesYes

Sturzstrom transport processes

The features listed in Table 2 are unique enough to establish that deposition was via sturzstroms. Unusual features such as jigsaw-puzzle fabric and preservation of bedrock domains after high-velocity, long-distance travel occur in both subaerial and submarine settings. The unusual features suggest unusual transport processes. Hypotheses seeking to explain sturzstrom movement must account for both subaerial and subaqueous settings thus rendering insignificant factors such as riding on a cushion of trapped air, heat at the base of the slide, or pressurized pore water in the substrate. The answers to the sturzstrom-flow process lie not below, but within the flowing mass itself.

Insight into sturzstrom flow has been provided by Melosh (1979, 1986) via the acoustic-fluidization hypothesis wherein trapped acoustic (vibrational) energy within a large mass of debris (> 106 m3) allows its rapid movement and long-distance travel. Melosh explains that the fall, shattering, and dispersion of a large rock volume creates acoustic waves, or vibrational energy, that reduces inter-particle friction and allows fast flow over a long distance. The existence of jigsaw-puzzle fabric and the preservation of bedrock domains shows that rock fragments tend to maintain their relative positions during flow and thus could propagate high-frequency sound waves generated within the thick mass of flowing debris.

Additional insight into sturzstrom flow can be derived from Campbell et al. (1995), whose experiments included a year-long computer run of a mass of 106 discs flowing down a virtual slope and onto a lowland. In the simulated flow of discs, particle collisions were most intense and the vibrational energy was greatest in the lowest portion of the flow, and supplied energy to help support the overlying mass. The resulting deposit of discs maintained the source distributions of colored discs analogous to sturzstrom deposits.

Conclusions

Sturzstroms are likely to occur where tectonic activity is creating: (1) steep slopes, (2) elevated, large masses of fractured rock, and (3) earthquakes to trigger initial falls.

Sturzstroms begin when fractured rock masses >1 × 106 m3 fall >300 m. Enough of the kinetic energy of the fall and shattering impact remain within the mass allowing flow distances >10 km. The secrets to the long runout must be sought within the flow, not the substrate.

Sturzstrom deposits may be recognized by their preservation of bedrock geometric relationships and stratigraphic order within the domains of shattered debris and by jigsaw-puzzle fabric.

Subaerial and submarine sturzstrom deposits contain the same sedimentary features, but differ in their basal contact relationships. Subaerial sturzstroms cut across the landscape decapitating stones that rise above the surface. Submarine sturzstroms have complex lower contacts due to injection and sinking of sturzstrom lobes and rising diapirs of basin sediments.

Paleoflow directions may be read from grooved and striated basal surfaces plus step-ups, ramps and crushed-rock streamers that rise upward in the down-flow direction.

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Acknowledgments

Our study was greatly aided by the help of others. We appreciate the financial support of a James Brainerd grant awarded by the Anza-Borrego Desert Natural History Association. Our investigation was aided by insights provided in the field at different times by Gordon Gastil, Mike Hart, and Jay Melosh. Our geochemical data benefited from the interpretive skills of Alice Cardenas. X-ray diffraction scans were run for us by Quincy Milton. Mike Walawender aided our understanding of the plutonic rocks and their geochemistry. An introduction to access in the southeastern part of the field area was provided by Park Rangers Paul Remeika and George Jefferson. The manuscript was significantly improved by the careful reading and helpful comments of Robert B. Johnson, William C. Haneberg, Steve Evans, and Jerry DeGraff. The final manuscript benefited from the actions of Abhijit Basu, Chuck Welby, and Jerry DeGraff. Tony Carrasco aided with the computer rendering of illustrations.

Figures & Tables

Figure 1.

Location map of the Fish Creek-Vallecito basin near the northwestern margin of the Salton Trough.

Figure 1.

Location map of the Fish Creek-Vallecito basin near the northwestern margin of the Salton Trough.

Figure 2.

Index map for the northern part of the Fish Creek-Vallecito basin. The Anza-Borrego Desert State Park administers most of the area. Exceptional and readily accessible exposures are found in Split Mountain Gorge.

Figure 2.

Index map for the northern part of the Fish Creek-Vallecito basin. The Anza-Borrego Desert State Park administers most of the area. Exceptional and readily accessible exposures are found in Split Mountain Gorge.

Figure 3.

Geologic map of the area in and to the east of Split Mountain Gorge. Modified from Kerr (1982).

Figure 3.

Geologic map of the area in and to the east of Split Mountain Gorge. Modified from Kerr (1982).

Figure 4.

Formal and informal stratigraphic nomenclature for the Neogene section along the western margin of the Saltan Trough.

Figure 4.

Formal and informal stratigraphic nomenclature for the Neogene section along the western margin of the Saltan Trough.

Figure 5.

View southeast of Miocene strata in 150 m high wall of Split Mountain Gorge. In lower left are alluvial-fan beds; in center is the red-and-gray sturzstrom deposit; at top, crossed by arrow, is vertical wall of Split Mountain megabreccia.

Figure 5.

View southeast of Miocene strata in 150 m high wall of Split Mountain Gorge. In lower left are alluvial-fan beds; in center is the red-and-gray sturzstrom deposit; at top, crossed by arrow, is vertical wall of Split Mountain megabreccia.

Figure 6.

Closer view of contact between red and gray sturzstrom mass and overlying Split Mountain sturzstrom deposit. Arrow points to 3 m-thick. sediment-filled stream channel formed between the two sturzstrom events.

Figure 6.

Closer view of contact between red and gray sturzstrom mass and overlying Split Mountain sturzstrom deposit. Arrow points to 3 m-thick. sediment-filled stream channel formed between the two sturzstrom events.

Figure 7.

Closer view of Figure 6 stream-channel deposit. Arrow points to 0.45 m-diameter boulder sliced in half by Split Mountain sturzstrom leaving a grooved surface on a decapitated stone.

Figure 7.

Closer view of Figure 6 stream-channel deposit. Arrow points to 0.45 m-diameter boulder sliced in half by Split Mountain sturzstrom leaving a grooved surface on a decapitated stone.

Figure 8.

Close-up view of Fish Creek Gypsum precipitated over boulders on top of Split Mountain sturzstrom deposit. Arrow points to coarse sandstone from subaerial fallout. Straw hat for scale.

Figure 8.

Close-up view of Fish Creek Gypsum precipitated over boulders on top of Split Mountain sturzstrom deposit. Arrow points to coarse sandstone from subaerial fallout. Straw hat for scale.

Figure 9.

View east up southwest-dipping stratigraphic surface on top of Split Mountain sturzstrom megabreccia. Note concentration of tonalite megaboulders on top of Split Mountain sturzstrom. On skyline to left is light-colored Fish Creek Gypsum deposited on Split Mountain sturzstrom. On right are marine fan-delta mud-stone and sandstone beds deposited on top of Split Mountain sturzstrom deposit. On upper right skyline atop the 240 m high slope is Pliocene submarine megabreccia.

Figure 9.

View east up southwest-dipping stratigraphic surface on top of Split Mountain sturzstrom megabreccia. Note concentration of tonalite megaboulders on top of Split Mountain sturzstrom. On skyline to left is light-colored Fish Creek Gypsum deposited on Split Mountain sturzstrom. On right are marine fan-delta mud-stone and sandstone beds deposited on top of Split Mountain sturzstrom deposit. On upper right skyline atop the 240 m high slope is Pliocene submarine megabreccia.

Figure 10.

Schematic paleogeographic map of the Split Mountain area in latest Miocene time. The Split Mountain sturzstrom began as a plutonic-rock fall on the steep face of the Vallecito Mountains, which shattered and flowed eastward down a large alluvial fan and then up a braided-stream valley around the Fish Creek Mountains.

Figure 10.

Schematic paleogeographic map of the Split Mountain area in latest Miocene time. The Split Mountain sturzstrom began as a plutonic-rock fall on the steep face of the Vallecito Mountains, which shattered and flowed eastward down a large alluvial fan and then up a braided-stream valley around the Fish Creek Mountains.

Figure 11.

Macroscopic grain-size trends in a vertical section through the Split Mountain sturzstrom deposit. Analysis made on photographs from stratigraphic section near peak 1704 (Fig. 2).

Figure 11.

Macroscopic grain-size trends in a vertical section through the Split Mountain sturzstrom deposit. Analysis made on photographs from stratigraphic section near peak 1704 (Fig. 2).

Figure 12.

A. Aerial photograph of a 4 km2 area of the upper surface of the Split Mountain Sturzstrom deposit; the Fish Creek Wash is to the left. B. The same image sharpened with the Finding-the-Edges filter to emphasize the megaclasts (>10 m diameter) exposed at the surface: there are ∼49 megaclasts per km2.

Figure 12.

A. Aerial photograph of a 4 km2 area of the upper surface of the Split Mountain Sturzstrom deposit; the Fish Creek Wash is to the left. B. The same image sharpened with the Finding-the-Edges filter to emphasize the megaclasts (>10 m diameter) exposed at the surface: there are ∼49 megaclasts per km2.

Figure 13.

Canyon-wall exposure ∼4 m high inside the Split Mountain sturzstrom deposit. Entire photo is biotite tonalite with one white pegmatite indicated by black arrows. In upper right is a tonalite boulder with the white pegmatite. The remainder of the tonalite, and the same white pegmatite, are shattered and display jigsaw-puzzle fabric.

Figure 13.

Canyon-wall exposure ∼4 m high inside the Split Mountain sturzstrom deposit. Entire photo is biotite tonalite with one white pegmatite indicated by black arrows. In upper right is a tonalite boulder with the white pegmatite. The remainder of the tonalite, and the same white pegmatite, are shattered and display jigsaw-puzzle fabric.

Figure 14.

Chemical compositions of a large tonalite clast and the nearby matrix in the Split Mountain sturzstrom deposit.

Figure 14.

Chemical compositions of a large tonalite clast and the nearby matrix in the Split Mountain sturzstrom deposit.

Figure 15.

A boulder protruding above the Miocene ground surface was overrun by the Split Mountain sturzstrom leaving a polished and grooved surface. Grooves are smooth toward the viewer and document a flow direction of 92°. The coin is 2.4 cm diameter.

Figure 15.

A boulder protruding above the Miocene ground surface was overrun by the Split Mountain sturzstrom leaving a polished and grooved surface. Grooves are smooth toward the viewer and document a flow direction of 92°. The coin is 2.4 cm diameter.

Figure 16.

Canyon-wall exposure cut through lower Split Mountain sturzstrom deposit. Shattered pegmatites step-up to the east (mimicked by black arrows), trending 105°. Staff is painted in decimeters.

Figure 16.

Canyon-wall exposure cut through lower Split Mountain sturzstrom deposit. Shattered pegmatites step-up to the east (mimicked by black arrows), trending 105°. Staff is painted in decimeters.

Figure 17.

Ramp within the lower Split Mountain sturzstrom deposit in distal reaches. Split Mountain sturzstrom debris on left stopped and was overrun by debris on right. Ramp rises to the east in the paleoflow direction.

Figure 17.

Ramp within the lower Split Mountain sturzstrom deposit in distal reaches. Split Mountain sturzstrom debris on left stopped and was overrun by debris on right. Ramp rises to the east in the paleoflow direction.

Figure 18.

Fish Creek sturzstrom megabreccia on top of fan-delta boulder-bearing sandstone beds at head of Split Mountain Gorge in 20 m high cliff.

Figure 18.

Fish Creek sturzstrom megabreccia on top of fan-delta boulder-bearing sandstone beds at head of Split Mountain Gorge in 20 m high cliff.

Figure 19.

Sketch of changes in Fish Creek sturzstrom morphology and bottom deformation from southeast (zone 1) to northwest (zone 4).

Figure 19.

Sketch of changes in Fish Creek sturzstrom morphology and bottom deformation from southeast (zone 1) to northwest (zone 4).

Figure 20.

Wall of Crazycline Canyon showing injected and sunk Fish Creek sturzstrom (FCS) breccia along each side of a large diapir of prodelta mudstone and sandstone. Outcrop is 35 m high.

Figure 20.

Wall of Crazycline Canyon showing injected and sunk Fish Creek sturzstrom (FCS) breccia along each side of a large diapir of prodelta mudstone and sandstone. Outcrop is 35 m high.

Figure 21.

Jigsaw-puzzle fabric within pegmatite and gneiss megaclasts at a Fish Creek sturzstrom domain boundary. Note the lack of mixing. Scale bar is in centimeters.

Figure 21.

Jigsaw-puzzle fabric within pegmatite and gneiss megaclasts at a Fish Creek sturzstrom domain boundary. Note the lack of mixing. Scale bar is in centimeters.

Figure 22.

Cliff wall at head of Split Mountain Gorge shows Fish Creek sturzstrom deposit on skyline with characteristic shattered, but not dispersed, white pegmatites.

Figure 22.

Cliff wall at head of Split Mountain Gorge shows Fish Creek sturzstrom deposit on skyline with characteristic shattered, but not dispersed, white pegmatites.

Figure 23.

Geochemical similarities between Fish Creek Sturzstrom bedrock clasts (plutonic, schist, pegmatite) and shattered matrix about one meter away from each respective clast. A scarcity of mixing is shown on a geochemical basis.

Figure 23.

Geochemical similarities between Fish Creek Sturzstrom bedrock clasts (plutonic, schist, pegmatite) and shattered matrix about one meter away from each respective clast. A scarcity of mixing is shown on a geochemical basis.

Table 1.

Matrix Geochemical Raw Data

SampleSiO2Al2O3TiO2FeOMnOCaOMgOK2ONa2OP2O5
Imperial Formation60.0418.400.745.160.040.733.033.821.900.17
    yellow mud
Fan-delta green55.7817.011.118.650.112.304.413.862.570.22
    mud
Split Mountain67.4015.870.563.400.063.571.823.143.440.12
    sturz. Matrix
Red and Grey67.8515.850.563.140.053.371.523.633.330.12
    sturz. Matrix
Braided stream56.1117.071.167.650.123.673.813.981.740.30
    red mud
Long Potrero67.3615.660.503.740.074.321.671.823.800.10
    tonalite
Morena Reservoir67.4015.540.554.180.104.081.091.264.740.15
    tonalite
SampleSiO2Al2O3TiO2FeOMnOCaOMgOK2ONa2OP2O5
Imperial Formation60.0418.400.745.160.040.733.033.821.900.17
    yellow mud
Fan-delta green55.7817.011.118.650.112.304.413.862.570.22
    mud
Split Mountain67.4015.870.563.400.063.571.823.143.440.12
    sturz. Matrix
Red and Grey67.8515.850.563.140.053.371.523.633.330.12
    sturz. Matrix
Braided stream56.1117.071.167.650.123.673.813.981.740.30
    red mud
Long Potrero67.3615.660.503.740.074.321.671.823.800.10
    tonalite
Morena Reservoir67.4015.540.554.180.104.081.091.264.740.15
    tonalite
Table 2.

Characteristics of Subaerial and Submarine Sturzstrom Deposits

FeatureSplit Mountain sturzstromFish Creek sturzstrom
Approx. volume300 × 106m3300 × 106m3
Lithologic domainsPervasivePervasive
Mixing w/in sturzstromNoNo
Preservation of bedrock relationsYesYes
Jigsaw-puzzle fabricAbundantAbundant
Shattered pegmatitesAbundantAbundant
Crushed-rock streamersCommonCommon
Decapitated stonesYesNot seen
Grooved & striated clastsYes, near baseNot seen
Shear surfaces within basal few mYesNot seen
Depth of substrate disturbance1 m>35 m
Folds in subjacent strataNoYes
Sandstone dikesYes, rareYes, esp. distally
Diapirs intruding sturzstromNoYes
Injected lobes of sturzstromNoYes
Megaclasts concentrated up topYesYes
FeatureSplit Mountain sturzstromFish Creek sturzstrom
Approx. volume300 × 106m3300 × 106m3
Lithologic domainsPervasivePervasive
Mixing w/in sturzstromNoNo
Preservation of bedrock relationsYesYes
Jigsaw-puzzle fabricAbundantAbundant
Shattered pegmatitesAbundantAbundant
Crushed-rock streamersCommonCommon
Decapitated stonesYesNot seen
Grooved & striated clastsYes, near baseNot seen
Shear surfaces within basal few mYesNot seen
Depth of substrate disturbance1 m>35 m
Folds in subjacent strataNoYes
Sandstone dikesYes, rareYes, esp. distally
Diapirs intruding sturzstromNoYes
Injected lobes of sturzstromNoYes
Megaclasts concentrated up topYesYes

Contents

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