Guide to a Waulsortian Buildup: Muleshoe Mound and Associated Strata; Sacramento Mountains, New Mexico, USA
Published:January 01, 2011
- PDF LinkChapter PDF
Kent C. Kirkby, Dave Hunt, J. A. Toni Simo, 2011. "Guide to a Waulsortian Buildup: Muleshoe Mound and Associated Strata; Sacramento Mountains, New Mexico, USA", The Carboniferous Geology of the Sacramento Mountains New Mexico, Robert J. Stanton, Jr.
Download citation file:
Muleshoe Mound is a composite Waulsortian buildup that forms a distinctive promontory along the western escarpment of the Sacramento Mountains (New Mexico, U.S.A.). Excellent exposures allow detailed study of the mound's fades, paleontology, geometry, internal architecture, and off mound stratal relationships. Recent work suggests that many Waulsortian mounds may have originated and grown in more agitated, shallower water environments than previously thought. The previously 'enigmatic' character of these Lower Carboniferous buildups may largely be due to differences in basin stratification, oxygen and possibly nutrient levels.
Muleshoe Mound can be separated into at least five distinct stratal packages or 'growth phases'. Successive growth phases differ in fades, geometry, and symmetry, reflecting different environmental conditions (energy, carbonate production, oxygenation and accommodation space). Intervening surfaces represent periods of hiatus and erosion. The architecture of this mound growth phase pattern strongly suggests that accommodation space was a critical control on mound growth.
Field study, serial slabbing, and petrographic examination of the mound facies indicate that Muleshoe grew in appreciable currents and intermittent high energy. Much of the micrite in the buildup originated as a microbial precipitation within an organic (algal?) host, rather than as depositional carbonate mud. This process of microbial precipitation, combined with extensive marine cementation, built the mounds' depositional relief and created a relatively rigid growth framework.
These features appear to conflict with the common interpretation of Waulsortian mounds as deep, quiet water buildups and Muleshoe may have formed in depths and energy conditions not dissimilar to many modern reefs. These features are not unique to Muleshoe, but occur in many Waulsortian mound suites. Differences between these ancient buildups and modern reefs suggest the modern ocean is an imperfect model for Early Carboniferous seas.
Although Muleshoe Mound is one of the most visited Waulsortian outcrops, the exposures retain a wealth of undiscovered detail. As succeeding generations of geologists 'rediscover' the mound, examining it from different perspectives, our understanding of Muleshoe and similar Waulsortian mounds is constantly evolving. This paper is presented in the form of a field guide to underscore our contention that the study of Muleshoe is still in an early stage. It is hoped that this guide might facilitate discussion, and encourage further investigations that will provide the next generation of insights/interpretation
Waulsortian mounds form a group of enigmatic carbonate buildups that flourished during the Early Carboniferous. The term 'Waulsortian' comes from the Belgium locality where Edouard Dupont first described examples of these carbonate buildups in 1883 (Lees 1988). These ancient carbonate buildups are quite distinct from modern reefs. Waulsortian mounds lack well-developed skeletal frameworks and are dominated by abundant micrite, and extensive submarine cements.
Ease of accessibility, excellent exposures and a dry climate have combined to make Muleshoe Mound the 'quintessential' North American Waulsortian mound. Studies of Muleshoe have played a major role in Waulsortian mound models (see history by Pray, 1982). Laudon and Bowsher (1941; 1949) published some of the earliest Muleshoe studies. They accurately reported the buildup's asymmetric geometry, and their illustrations and discussion suggest a relatively shallow water environment. Pray (1969) identified the important role that submarine cementation played in Muleshoe's growth. He proposed a deep, quiet water origin for the mounds, but allowed for their upward growth into shallow, agitated water. Lees, Hallet and Hibo (1985) identified four component assemblages in Belgium Waulsortian mounds that were interpreted to reflect different depth zonations. One assemblage contains recognizably photic organisms and abundant micritized grains, and is interpreted to have formed within the photic zone. The remaining assemblages (present at Muleshoe) were interpreted to be subphotic with assumed water depths in excess of 100 meters. Lees and Miller (1985) expanded this work to a number of European Waulsortian localities. These papers had a strong influence on subsequent mound studies. In the following decade most Waulsortian mounds, and many other 'mud mounds', were interpreted as deep, quiet water buildups.
Previous Muleshoe studies focused on the buildup's paleontology and large-scale stratigraphic relationships. In contrast, our work emphasized internal stratal patterns, mound-to-flank transects, and the mound fades' megascopic textures. This perspective led to the recognition of characters that appear to conflict with a deep water interpretation.
Muleshoe Mound and correlative strata form part of a Lower Carboniferous (Tournaisian-Viséan) carbonate ramp sequence - the Lake Valley Formation (Cope 1882; Fig. 1). The western escarpment of the Sacramento Mountains contains a spectacular, relatively undeformed, exposed dip (or oblique dip) section of the Lake Valley Formation from shelf deposits in the north (Indian Wells Canyon) to deeper, more open basin deposits in the south (San Andres Canyon and beyond; Fig. 2). Four members of the Lake Valley Formation contain or are correlative with known Waulsortian buildups: the Alamogordo, Nunn, Tierra Blanca, and Table Top Members (Pray, 1961; De Keyser, 1983; Lane, 1982). Of these, the Alamogordo Member probably constitutes a chronostratigraphic unit, while the others are lithostratigraphic units with only limited biostratigraphic control (Lane 1982).
Like many Lake Valley buildups, Muleshoe Mound began its growth during deposition of the Alamogordo member. Initial mound growth appears to have colonized antecedent relief generated by deposition, compaction, and localized tectonic processes (Ahr 1989; Ahr and Stanton, 1992; Hunt 1994; Hunt et al. 1995). The Alamogordo Member is distinguished from subsequent members by its remarkable regional uniformity and the fossiliferous nature of its intermound strata. Although Alamogordo mounds thicken slightly towards the basin the unit forms a relatively undifferentiated ramp, with no evidence of coeval anoxic bottom waters (Jeffery and Stanton, 1993).
Subsequent strata were differentiated into distinct 'shelf' and 'basin' components with a tendency toward episodically-anoxic bottom water in the basinal portions of the ramp. In the north, a north-south trending elongate chain of composite Waulsortian mounds (Alamogordo-Tierra Blanca intervals) acted as a rampart to deflect and focus currents towards the south (Hunt and Allsop 1993; Hunt 1994; Hunt et al. 1994, 1995). These currents in turn controlled the development of a large (9km long by 3-5 km wide) lobe of crinoid-dominated skeletal sands (Fig. 2) - the Tierra Blanca 'shelf' of Meyers (1975). Mound growth within the confines of this lobe appears to reflect an accommodation control as mounds built up to a common level. These mounds tend to be elongate, current-scoured, and exhibit strong lateral progradation on their southern flanks (Fig. 3).
South (basinward) of this platform, a suite of relatively large (80-100 meter relief) buildups developed along a subtle toe of slope break with an average spacing of 1.6 km. No known mounds exist farther basinward. Vertical aggradation dominated the early growth of these Lake Valley buildups, while later phases were characterized by progradation. Total aggradation of these mounds (-100 meters) is similar to that of the time-equivalent Tierra Blanca lobe to the north, suggesting a regional accommodation control; we speculate that this may represent wave base.
Muleshoe Mound sits at the head of a small ridge bordering San Andres canyon in the Sacramento Mountains of New Mexico (se nw sec. 28, T17S R10E; Fig. 2). Muleshoe is 110 meters high, with a width of 400-500 meters. Recent erosion exhumed three fourths of the mound, resulting in a close-to-center exposure through the buildup core (SW face), and two mound core-to-flank sections on the shelfward (N) and basinward (SE) sides of the buildup (Fig. 8).
Pray (1975) subdivided Muleshoe into informal lower and upper mound intervals. Kirkby and Hunt (1996) further divided Pray's upper mound interval into four growth phases based on stratal architecture and truncation surfaces (Fig. 9). Some of the growth phases at Muleshoe are clearly composite units that include multiple sets (possibly cycles) of strata. At present, these smaller sets aren't considered to be separate growth phases, as it could not be determined that their boundaries represented significant hiatuses.
Waulsortian facies have been traditionally divided into two end-member groups: massive 'mound' facies - here interpreted to have grown largely by biologic and chemical precipitation of micrite and cement; and bedded 'flank' facies that accumulated as detrital grains and mud. A 'mound' and 'flank' terminology is somewhat misleading though. Detrital carbonate sands can occur throughout intervals of mound facies, representing fluctuations in the mound community and/or energy conditions. The abrupt transition between massive and bedded strata along Muleshoe's younger slopes appears to be an artifact of deposition in a well-stratified basin. Massive mound facies grew in well-oxygenated waters while downslope strata consist of starved anoxic intermound strata and resedimented grain flows that cascaded down the steeply dipping buildup slopes. In older stages of mound growth, the transition between intercalated mound and flank strata is far more gradational.
Mound facies in Waulsortian mounds are characterized by a massive, unbedded appearance, and abundant cement and micrite. On outcrop surfaces, the micrite and cement components are segregated into two anastomosing networks that create a distinctive 'mottled' or 'thrombolitic' appearance (Fig. 4 a). Cements typically consist of multiple isopachous rims of cloudy (inclusion-rich), fibrous-to-bladed crystals that may have associated grainstone fills. Extensive marine cementation of the mound strata had a strong influence on its subsequent modification (dike formation, breccias, differential compaction, and diagenesis).
Although Muleshoe is commonly referred to as a 'mud-mound', the terms 'micrite mound' or 'micrite reef' are more appropriate. Most of the micrite has a clotted microscopic texture (Fig. 4 b), and is interpreted to have resulted from in-situ microbial precipitation within a soft-bodied, loosely organized (algal or sponge?) host that was capable of building synoptic relief. This process of microbial precipitation, combined with marine cementation, built the mound's vertical relief and provided it with a resistant growth framework.
'Flank' facies are here limited to the well-bedded grainy strata that drape the buildup slopes. They were deposited as grain flows and resedimented deposits that originated higher on the buildup slopes. At Muleshoe Mound, most flank material was generated within the buildup complex and has only been locally reworked. A crinoid-dominated community grew on the mound slopes and intermittently colonized the mound crest. Debris from this community was easily remobilized, cascading downslope as debris flows or swept off the mound by storms. Breccia and clast-bearing deposits are a distinctive component of the flank facies. Muleshoe's breccias are composed of subrounded to angular clasts of core facies in a crinoidal matrix. Truncation of early-marine cements at the clast boundaries indicates these clasts were lithified prior to their erosion (Pray 1969; Meyers 1975).
Stratal relationships, such as those seen in (c) and (d), led to the realization that growth phase boundaries represent significant hiatal breaks. This interpretation was further supported by recognition that the flank strata of each growth phase have slightly different cementation histories (see Figure 6).
Ramp and Basin Facies
Intermound strata correlative to Muleshoe consist of a lower set of normalmarine basin mudstones/wackestones that are lateral equivalents of the first growth phase (Alamogordo Member) unconformably overlain by dark, laminated mudstones (Table Top Member) that are equivalent to the youngest Muleshoe growth phases (IV and V). Hiatus and erosion surfaces form the only lateral equivalents of the intermediate growth phases (II and III). After deposition of the fifth growth phase, Muleshoe was onlapped and encased in dark, laminated, anaerobic-to-dysaerobic lime mudstones (Arcente Formation) that are iithologically indistinguishable from the Table Top Member.
Muleshoe Mound can be separated into at least five distinct stratal packages or 'growth phases'. Field and diagenetic study of their bounding surfaces indicate that successive growth phases were not part of a depositional continuum. Rather, each growth phase was a separate recolonization of the mound topography after a prolonged period of hiatus and erosion. Such episodic mound growth distinguishes these growth phases from other reef subdivisions that reflect gradational environmental change.
Muleshoe's growth phases were initially recognized and distinguished by variations in core geometry and flank stratal patterns. Stratal truncations occur at the top of growth phases and beds of the succeeding package downlap onto these truncation surfaces (Fig. 4 c and d). Flank strata of successive growth phases also exhibit marked changes in bedding orientation (Fig. 5). Boundaries between growth phases are sharp, scoured surfaces that are locally overlain by breccias. The distribution of megabreccia and breccia units is not random, as previously thought, but is associated with the hiatal surfaces. Erosion, stratal patterns, and diagenetic evidence combine to show that the growth phases' bounding surfaces represent significant hiatal breaks. Further, syntaxial overgrowths on crinoid grains in flank strata of growth phases III to V exhibit different luminescent patterns (Kirkby, 1994). Our work confirms Meyers' earlier (1974) observation that three cement zones are present in the Muleshoe strata (Meyers' zones 1, 2, and 5), but also recognized that the distribution of these zones reflects the mound's subdivision into growth phases. Crinoid grainstones in growth phase III exhibit all three cement zones, and have composite zone 1 cements (consisting of at least two subzones; Figs. 6 a & b). In growth phase IV, zone 1 cements typically consist of a single homogeneous zone (Figs. 6 c & d). Cement zones 1 and 2 are not present in growth phase V (Figs. 6 e & f), and cement zone 5 is the only pore-filling cement present. We interpret this pattern of cements to reflect partial cementation of flank strata during periods of hiatus.
Muleshoe Mound provides a case study of episodic growth in Waulsortian mounds. But episodic growth may be a common component of many Paleozoic buildups. Recognizing episodic growth is important as growth phases can provide a finer temporal framework than available biostratigraphy. Growth phases also provide an internal buildup stratigraphy for fauna and textural study, or predicting subsequent reservoir development and partitioning. The cumulative pattern of growth patterns and intervening surfaces form a record of the basin's physical evolution that is not often recognized in the starved intermound sections.
Growth Phase I
The oldest growth phase revealed in Muleshoe's exposures is a broad, relatively low relief (35-40 m high, 200 m wide) buildup - synonymous with Pray's (1975) 'lower mound' (Figs. 1, 9 and 10). Massive micrite-rich mound facies grade laterally into normal-marine off-mound skeletal wackestones and mudstones (Alamogordo Member) with no intervening grainy flank facies. A sharp, irregular, and locally erosive surface separates the relatively massive mound facies of growth phase I from overlying thin-bedded crinoidal grainstones and packstones (Fig. 10). The upper ten meters of growth phase I is bounded by this erosion surface. Below this level, the surface was truncated by erosion that occurred in association with a depositional hiatus at the end of the second growth phase.
Growth Phase II
Growth phase II unconformably succeeds its precursor and consists of an areally-restricted building up of the mound's relief (to a cumulative height of -70 m and 120 m width in the exposed face). Exposed outcrops consist of intercalated lens of massive mound facies and bedded flank facies with little lateral segregation of mound and flank strata (Figs. 9 and 10). Gently dipping beds (5° - 10°) at the base of the growth phase grade up into more steeply dipping strata (25° to >45°) that were locally deformed (towards the toe-of-slope) in association with the hiatus following growth phase II.
Growth phases II and III are separated by a sharp surface that makes a pronounced recessive break in the outcrop face (Fig. 10). This surface erodes the top of growth phase II, the flanks of growth phases I and II, and has also removed the youngest inter-mound Alamogordo strata up to 150 meters from the buildup, over which an extensive sheet-like breccia occurs (Fig. 9). Mound strata of growth phase III downlap onto this eroded surface and small clasts of growth phase I and II mound facies are incorporated into the basal flank facies of growth phase III.
Growth Phase III
The third growth phase marks a significant change in the buildup's growth from aggradational to progradational stratal patterns (Figs. 9, 10, 11 & 12). Growth phase III consists of stacked, massive mound facies that laterally grade abruptly into thin-bedded flank strata. Mound height increased during growth phase 111, but most growth was progradational. At the end of growth phase III the mound complex formed a roughly symmetrical dome over 110 meters high and 300 meters across.
Mound strata in growth phase III exhibit distinct lateral facies transitions. Central mound facies are only accessible in the buildup's upper exposures. These consist of digitate, subvertical micrite masses with intervening aligned in-situ fenestrate bryozoans. Micrite masses and bryozoan fronds are both encased in thick rims of isopachous marine cement (Fig. 7a). Skeletal grainstones (crinoid and broken fenestrate bryozoan fronds) ring the central cementstone mass. Towards the core-margin these are succeeded by micrite-rich skeletal packstones with a silicified fenestrate bryozoan-rich fauna that includes large cylinder- and vase-shaped bryozoan growth forms (Fig. 7b). These core margin strata overlie and pass into steeply-dipping (to 35°) flank strata. They comprise a previously unrecognized, unstable depositional facies, distinguished by multiple generations of neptunian dikes, and an in-situ 'brecciated' fabric with large (meters-scale) rotated blocks of micrite-rich mound facies. Proximal flank strata consist of massive beds of crinoid-rich packstones with small stromatactis features. These grade abruptly into thin-bedded flank facies. The transition is interpreted to reflect an oxygen pycnocline. The bedded flank strata consists of alternating anaerobic mudstones and crinoid debris flows. Burrowing is limited in these strata, but abundant in the adjacent proximal flank strata, and the massive character of the latter is attributed to bioturbation.
The boundary between growth phases III and IV is well exposed on Muleshoe's southern side. Flank strata of growth phase III are truncated by an erosive surface (Fig. 4d). Spectacular megabreccias overlie this surface, mantling and deforming the lower buildup slope and adjacent 'level-bottom' strata. These megabreccias typically are composite units, deposited by several individual flows. They contain blocks of core and core margin facies in a crinoid-rich matrix. Those on the south (basinward) side of the buildup (Fig. 11) were initially reported by Pray (1958) and described in detail by Meyers (1975). A large channel complex (up to 15 meters thick) and associated megabreccia deposits also occur on Muleshoe's northern flank (Kirkby 1994; Fig. 12). Megabreccia deposits on both sides of the buildup appear to be roughly correlative.
Growth Phase IV
In contrast to the symmetrical progradation of growth phase III, progradation in growth phase IV is strikingly asymmetric. Preserved mound strata are only developed on the southern (basinward) side of Muleshoe Mound, where they form three overstepping lenses that intercalate, and abruptly pass into, bedded flank strata. The contact between successive mound lenses and their associated flank strata rises towards the basin (Figs. 9 and 12). Correlative strata on the northern flank are composed entirely of bedded flank facies.
Erosion related to Late Carboniferous subaerial exposure and Muleshoe's recent exhumation have removed the mound crest strata of this growth phase. Preserved mound facies exhibit lateral facies tracts similar to those of the underlying growth phase. Facies asymmetry also occurs in the bedded flank strata. Grain-supported strata dominate the southern (basinward) side of the buildup, while matrix-supported beds characterize the northern flank (Fig. 13). Indications of depositional energy (scour, graded beds, and small intraclasts) are also more abundant in the southern flank deposits. The combination of flank and mound asymmetry suggests selective mound growth under the influence of current and/or wave energy.
The surface capping growth phase IV is more subtle than the other hiatal surfaces, as relatively little erosion of underlying strata occurred. A clast-bearing interval was deposited on both sides of the mound (Fig. 9), and is down-lapped by flank strata of the succeeding growth phase.
Growth Phase V
The youngest growth phase (V) is an asymmetrically prograding package with a single lens of mound facies on the southern side of the mound complex. This lens is downset from the buildup crest and is characterized by a basinward descending mound-flank transition (Figs. 9 and 11). Crest deposits were eroded and the northern flank is entirely composed of bedded flank strata. Mound and flank facies are similar to those in the underlying growth phase. Growth phases V is distinguished solely on the basis of downlapping stratal patterns and cement history.
Flank strata of growth phase V are either truncated by the overlying surface, or rapidly grade into restricted, laminated strata of the off-mound Table Top Member. Surface exposures at Muleshoe are not conclusive. However Table Top and Arcente Formation strata do intercalate mound flank strata in Alamo Canyon, seven kilometers north of Muleshoe (Meyers - pers. com. 1993; Hunt and Allsop 1994; Hunt 1994). This relationship implies the presence of a pronounced basin oxygen-stratification coincident with mound growth.
The growth of Muleshoe Mound terminated as it was smothered by a thickening toe-of-slope wedge of lime mud deposited under low-energy, oxygen-poor (anoxic) conditions. 'Drowning' of both the Lake Valley platform and buildups is recorded by strata of the Arcente Formation (Fig. 11). These strata are dark lime mudstones and interbedded calcareous shales with a sparse fauna and rare Chondrites. Lap-out of these strata against the topography of both the platform and its buildups indicate a reduction of water energy and oxygenation related to relative sea level rise and/or nutrient upwelling.
Muleshoe's topographic relief continued to influence subsequent deposition. Allodapic sheets of crinoidal grainstone occur in the Arcente Formation that indicate Muleshoe's crest supported an intermittent, normal-marine, benthic community. During deposition of the succeeding Dona Ana Formation, mound core facies developed over Muleshoe, and beds of skeletal debris prograded from Muleshoe, thinning into the surrounding basin. Following Dona Ana deposition, Muleshoe Mound was exhumed and eroded prior to deposition of both the Rancheria Formation, and Upper Carboniferous Gobbler Formation.
Muleshoe can be reached from the Taylor Ranch southeast of Alamogordo. Be sure to stop and request permission before crossing private ranch land. The path from the valley floor up to Muleshoe crosses strata from Ordovician to Early Carboniferous age. Above the Dona Ana and Rancheria carbonates, there is a thick section of recessive Gobbler Formation (Upper Carboniferous ) strata. The imposing carbonate cliffs that form the escarpment's skyline compose the Bugscuffle Limestone member of the Gobbler Formation. Muleshe Mound is not the only large Lake Valley mound seen here. A large well-exposed buildup (Sugarloaf) forms a promontory well to the south while another, less-well exposed, buildup occurs in the canyon wall south of Muleshoe.
The climb up to Muleshoe Mound is fairly rigorous and crosses rough terrain. Visitors should take proper precautions, especially on hot days. Be sure to bring 2-3 liters of water, sun protection, and some means of attracting attention in case of an emergency - such as whistle or cell phone. Carrying a small medical kit is a good idea, if for no other reason than to remove the occasional cactus needle. If you do decide to take the trek alone, be sure to notify someone of your estimated time of return. The large number of visitors to Muleshoe Mound has already left its mark on the outcrops. Please respect the outcrops and try not to leave any trace of your passage (graffiti, litter etc.). The next generation of geologists will appreciate your efforts.
Before approaching Muleshoe, take time to observe it from the valley floor or from the top of the small knoll southwest of the buildup. Once at the mound, it is difficult to step back to see large scale relationships. Compare the outcrops with the growth phase division given in Figures 9, 10, 11, and 12. When you have a visualized the buildup geometry and master bedding planes, proceed to the first stop. The easiest approach is to climb towards the southeast flank of Muleshoe and contour back along the mound base or top of Alamogordo. However, if you're in reasonable shape, the direct climb to the base of the southwest face is much faster. Station locations are shown on figures 10 to 12. The station order was chosen to accommodate logistics as well as stratigraphic succession.
This face provides the best exposures of 'classic' micrite-rich Waulsortian mound facies developed in the lower mound (growth phase I). Mound strata exhibit a distinctive macroscopic mottled texture (Fig. 4a) consisting of masses of fossiliferous micrite with intervening cement-filled areas (previously termed stromatactis cavities). While burrows (Tedesco and Wanless, 1988) and decayed organic matter (Bourque and Boulvain, 1993) may have formed some of these voids, we interpret most of them to have originated as the intervening open space between thrombolitic masses of microbially precipitated micrite (Kirkby et al., 1996a, 1996b). These cavities were largely filled by isopachous layers of fibrous-to-bladed marine cements, but some were filled by later sediment that filtered down (up to 1.5 meters) from the growing mound surface. Large, erect, in-situ bryozoan fronds occur in many of these cavities. Fronds were often partially incorporated into the micrite mass and marine cements line both the fronds and the micrite masses.
Although a microbial origin for micrite in Waulsortian buildups has been proposed, models tend to be low relief biofilms (Lees and Miller, 1995), or incomplete algal mats (Pratt, 1982). Microbial precipitation in Muleshoe exhibits a well-developed thrombolitic character. Micrite masses in this lower growth phase have rounded, bulbous shapes, with no evidence of current alignment. However, episodic energy is indicated by a number of erosionally-based crinoid grainstone layers interspersed through the mound strata, and by grainstone fillings of some mound voids.
Erosional surface between growth phases I and II. Surface truncates several meters of the underlying strata. Argillaceous sediment drapes much of the surface and can be seen to filter down into cavities in the underlying growth phase. The surface and shale drape represent a period of mound hiatus and erosion.
Growth phase II was an areally restricted strongly aggradational buildup phase that built up a minimum of 40 meters depositional relief. Depositional energy during this growth phase is evidenced by the abundance of grainstone layers and even a small channel feature above the top of the exposed surface. These beds represent the buildup's lower grain-rich margin as the cliff face provides a flanking cut through the buildup, not a central cross-section. Muleshoe's second growth phase 'grew' so quickly that its margin slopes were relatively unstable. On both sides of this station, lens of micrite-rich mound facies are crumpled into folds along their lower slope.
From Station 2 walk along the base of the cliff towards the northwest. Note that the bedding steepens as you approach the contact between growth phases II and III.
Along the base of the cliff , you can see darker, thicker beds of Waulsortian facies interbedded within the grainy crinoidal 'flank' strata and recessively weathered argillaceous partitions. These layers are typically lensoid, many with shape eroded and erosive boundaries. Some of the thicker lenses exhibit rapid facies transitions from grainy to micrite-rich facies, and are cut by limestone dikes. These micrite-rich layers are interpreted to be detached fragments of an inherently unstable transition zone and have crept or been redeposited downslope where they are enclosed within the more typical flank facies. Micrite-rich layers increase in number and thickness towards the top of the preserved growth phase II strata. Some of these exhibit mottled textures similar to those seen at Station 1. The phase ll-lll boundary truncates the underlying strata and is locally overlain by breccia deposits. A drape of weathered argillaceous material forms a recessive that highlights this contact at a distance.
Proceed around the corner to the southernmost part of Muleshoe's northwest face.
This is the southernmost of two eroded 'windows' of growth phase II strata on the northwest face. Note the downlap of phase III beds onto the underlying eroded phase II strata. Growth phase II strata at this station are dominantly micrite- and cement-rich Waulsortian facies, in contrast to the bedded 'flank' facies that characterized growth phase II strata on the southwest face. Numerous neptunian dikes cut these strata as well as the overlying growth phase III strata. Some of these dikes were defaced by a British geologist who shall remain nameless.
Walk back and examine the exposures at the extreme southwest corner of Muleshoe
Be careful. As in many parts of Muleshoe, it is difficult here to get a good sense of stratal geometry without inadvertently killing yourself. These exposures contain early-deformed strata, as well as a later tectonic overprint associated with recent uplift. These deformed strata have been interpreted as partial evidence for an allochothonous origin for Muleshoe Mound (Giles, 1994 ; 1995). However, we believe Muleshoe is an in-situ carbonate buildup and suggest this deformation may have been generated by emplacement of now-eroded breccia flows, similar to those seen at stops 9 and 20.
Walk back to the southeast towards the exposure of the growth phases II/III contact east of station 1. Along the way note the steeper dips present in the lower phase II strata on the south side of the mound, compared to those on the north side. Note also the sigmoidal progradation of the bedding towards the southeast. As you approach the contact notice the striking number of neptunian dikes and a number of folds due to downslope creep. Scan the float blocks for some spectacular examples of multiple-generation lime-mud filled dikes.
Strata and erosional surface at growth phase II/III boundary. The boundary sharply truncates the underlying strata and forms a deep recessive in the exposures (possibly due to erosion of an argillaceous drape). Small folds and multiple generations of carbonate-filled fractures (neptunian dikes) occur in the underlying phase II strata. Folds and dikes reflect downslope creep of the steep mound margin facies. Tilted geopetals within these beds suggest the mound margin was steepened by differential compaction of interbedded grainy and argillaceous flank beds relative to the cement- and micrite-rich mound facies. This unstable margin was the source area for breccias that overlie the Alamogordo (growth phase I lateral equivalents) at station 10.
Stations 7 and 8 are encountered as you continue to the southeast.
These massive facies formed as toe-of-slope deposits laterally equivalent to the microbial mound crest facies of Station 16. Much of the micrite present in these downslope facies originated as depositional mud in relatively quiet water. As you move to the southeast, note the abrupt transitions between massive and bedded flank strata. Bedded sets of intercalated grainstone and argillaceous layers grade abruptly updip into massive mound facies. The sharp gradation from mound to flank strata is not a weathering artifact, but a reflection of a depositional pycnocline between well oxygenated and dysaerobic/anaerobic waters. Massive mound strata were extensively burrowed, while intermound bedded strata remained undisturbed. The well-bedded crinoid layers contain rounded micrite intraclasts and exhibit graded bedding and partial Bouma sequences. They are interpreted to represent grain flows (generated by storms or margin collapse) that intermittently cascaded down into oxygen-starved bottom waters. Later, downward fluctuations of the oxygenated water column allowed bioturbation to obscure the original transition between mound and intermound strata.
Asymmetric folds in bedded flank facies. Similar, although less intense folds also occurred at the phase ll/lll boundary. These folds resulted from compression at the toe-of-slope, and are associated with a downwards thickening of massive facies towards the mound-flank transition.
Proceed along the base of the cliff southeast towards the major megabreccia unit that marks the phase 11 l/l V boundary at the mound-flank margin.
At this station it is easy to look back and appreciate the stratigraphic condensation of growth phases ll-lll. Over 70 meters of mound strata thins laterally to less than 4 meters of bedded flank strata. The boundary between growth phases III and IV is marked by a major megabreccia interval. Previous to Pray (1958) this megabreccia lens had been interpreted to be a satellite buildup (an interpretation that occasionally resurfaces in the literature; (Scholle, 1983). This is a composite megabreccia interval, consisting of three distinct breccia lens, separated by inversely graded beds. Two megabreccia lens are present at this station, a third lens truncates the Alamogordo section at Station 10). The two breccia units at this station truncate and deform the underlying phase III flank strata and are in turn draped by overlying phase IV flank strata. Less obvious is a 70 degree change in dip directions between the flank strata below and above this contact (Fig. 5).
During a cessation of active mound growth, differential compaction of flank and intermound strata would result as cement-rich microbial mound strata were more resistant to compaction. This compaction steepened the buildup margin slopes and further increased slope instability. Megabreccias at this station are interpreted to have formed from the collapse of unstable mound margins during the hiatus between growth phases III and IV. Megabreccia units along that toe-of-slope of Waulsortian mounds are common, though seldom noted in the literature. Every large Waulsortian mound exposed in the Lake Valley has at least one and often multiple resedimented and displaced breccia units on its flanks. Similar breccias are also seen in subsurface Waulsortian mounds in the Pekisko Formation of Alberta (Kirkby, 1994).
Stations 10 and 11 drop down stratigraphically to the Alamogordo and Andrecito units.
This is a smaller Alamogordo buildup that is laterally equivalent to Muleshoe's growth phase I. Unlike Muleshoe, it was not recolonized during the deposition of growth phase II, possibly because it was below a critical threshold (oxygen pycnocline?). Instead this mound was eroded and truncated by debris flows associated with growth phases III and IV. North of this mound, the intermound Alamogordo strata are unconformably overlain by a breccia unit that can be laterally traced to Muleshoe's growth phase ll/lll boundary. Differential compaction of the growth phase III beds that draped the mound reversed their original orientation. These flank beds now dip back towards Muleshoe (these compacted dips were not included in Figure 5). Growth phase IV flank beds truncate these beds at the mound crest and also removed the southern mound slope. Some phase III beds are partially silicified at this angular unconformity. Silicification decreases away from the eroded surface and is interpreted to have been a sea-floor alteration during the hiatus between growth phases III and IV. Meyers (1977) examined silicification of Lake Valley strata and determined that the mobilization of silica from opaline sponge spicules was an early diagenetic process.
Proceed southeast to the gully and drop down to below the Alamogordo level.
This is the lowest exposed Lower Carboniferous strata of this transect. This is a channelized grain-rich bundle of Andrecito strata. Such 'skeletal pods' (Meyers, 1978) are thought to have developed a depositional topography that acted as a substrate for subsequent Alamogordo mound growth. Ahr (1989) interprets these 'pods' to have formed from sediment baffling by crinoid thickets. On the basis of cross bedding and stratal geometry, we suggest they are channels that developed depositional relief through the differential compaction of less grain-rich lateral equivalents. A direct tie of this local topography to subsequent mound growth is difficult to prove as the base of the Waulsortian mounds is seldom exposed, and central cuts through the buildups are even more rare. This exposure is one of the best examples of the transition from Andrecito strata up into an Alamogordo mound.
As you go back up the gully, the Alamogordo (growth phase I equivalents) are truncated and unconformably overlain by flank deposits of growth phase IV. On the southern side of the gully a third lens of megabreccia forms the massive beds. This is a lateral equivalent of the megabreccia units seen at Station 9. Above this level note many excellent exposures of redeposited flank beds. During growth phase IV and V, grain-supported strata was volumetrically more important on this basinward side of Muleshoe than on its northern flank (Figure 13).
Cimb up the gully to a prominent breccia bed with many voids (weathered-out intraclasts). A large overhang of this layer provides one of the few opportunities for shaded discussion or a late lunch. Station 12 is located in the gully above this layer.
This is one of the better exposures of the transition into basin beds seen at Muleshoe. These Table Top strata are dark, thinly bedded, laminated lime mudstones with a noticeable lack of bioturbation. They are interpreted to be have been deposited in dysaerobic to largely anaerobic conditions. Although similar facies appear to intercalate mound flank strata along Waulsortian mounds at Alamo Canyon, it is difficult to determine the relationships between mound and basin strata in these exposures. However, at least intermittent normal water conditions and an active coeval carbonate community on Muleshoe's crest are indicated by crinoid debris beds that interrupt this succession of Table Top basin strata. Thin, reddened horizons occur above these calciturbidites that contain an abundant fenestellid bryozoan fauna, and burrows penetrate down from these surfaces into the calciturbidites. The general absence of bioturbation and fauna in the 'normal' basin beds is interpreted to record the development of relatively anoxic, stagnant bottom waters. These conditions were temporarily interrupted by mixing of waters associated with turbidity currents.
Dark basin strata have been interpreted as evidence for the mound's deep water origin, however these characteristics reflect oxygenation, not necessarily depth. An association of Waulsortian mounds and anoxic bottom waters is common in many Early Carboniferous basins, including basins where it is geometrically difficult to evoke great depths. We suggest that this relationship of mounds and anoxic bottom waters may reflect higher nutrient levels and a more pronounce stratification of Early Carboniferous seas. Higher nutrient levels tied to proximity to anoxic bottom waters could have decreased faunal diversity and resulted in the mounds' microbial character. Carbonate buildups in a less nutrient-rich setting would probably have developed a more diverse fauna.
Contour across the top of the underlying flank crinoidal strata and up the small gully on Muleshoe's southwest flank (this gully can also be reached by climbing across the megabreccia beds at station 9). As you climb up through thin-bedded, non-fossiliferous Table Top strata, you'll encounter a more thickly bedded, resistant interval of dark, cherty, thickly bedded lime wackestones with a diverse fauna.
These are the informal Hackberry beds of Hunt et al. (1995). Beds are nodular, but generally continuous. The fauna is dominated by fenestrate bryozoans (some upright and in growth positions) but rugose corals and brachiopods are common. Abundant Chondrites burrows occur in the argillaceous partitions between beds. These beds high on Muleshoe's slope are notably richer in fauna than their lateral equivalents downslope, suggesting more oxygen-rich, normal marine waters associated with Muleshoe's topographic relief. The Hackberry beds are interpreted to represent a regional fall in the boundary between anoxic and oxic waters, or an 'oxic-event' in the otherwise anoxic bottom water record (basin overturn). Note that these beds are disturbed. Hackberry strata buckled and slid off Muleshoe due to differential compaction of the basin mudstones during and after Arcente deposition. Looking north or south from Muleshoe's crest, resistant Hackberry layers can be seen to form a series of folds detached from Muleshoe's slopes.
Contour up and across Muleshoe's slope to the massive lens of growth phases IV and V.
These massive lens are the only mound strata of growth phases IV and V. On the northern (shoreward) side of the buildup, preserved lateral equivalents are entirely bedded flank strata. Note the textures in the mound facies and compare them with those of growth phases I and II. These mounds intercalate and overlie compactable bedded flank strata. Their unstable substrate is reflected by the abundant cross-cutting sets of neptunian dikes, often exhibiting multiple fills. Packages of Waulsortian facies on the top and northern side of these lens dip back towards Muleshoe's bulk.
Continue up towards the mound margin strata of growth phase III.
Etched 'windows' of growth phase III upper mound margin strata. These strata are broken by multiple generations of cross-cutting neptunian dikes. Large blocks of the margin strata (to 10 meters in diameter) are rotated up to fifty degrees towards the basin slopes. This area is a beautifully preserved, in-situ example of an unstable mound margin at the top of a relatively high angle buildup slope. Storms, seismic events, or continued steepening of the slope during periods of hiatus could have triggered mass wasting of the margin by differential compaction. The common occurrence of clast-bearing intervals in flank deposits attests to the ongoing deconstruction of the buildup margin during deposition of the upper growth phases. During periods of mound growth hiatus, the concentration of these processes led to the development of the megabreccia units that punctuate growth phase boundaries.
Continue up to a point on the massive shoulder of Muleshoe where you can see the large-scale geometric relationships of the post-Muleshoe strata.
This station provides an opportunity to see the mound facies of the upper growth phase III, some spectacular limestone dikes and an excellent overview of the post-Muleshoe stratigraphic relationships. There are a number of naturally etched 'windows' in the rock where you can find the thrombolitic texture of these upper mound strata beautifully displayed. These are the central and upper mound facies of growth phase III that are laterally equivalent to the massive mound/flank transition beds seen at station 7 nearly 80 meters downslope. Microbial micrite forms distinct oval and circular masses that are lined by isopachous layers of early marine cements. In core and vertical serial slabs, the micrite masses are digitate in shape and grew at a relatively high angle. Consistent internal geopetals indicate that this texture is depositional and not a product of rotation or allochthonous origin (Kirkby et al., 1996b). Coarse unbroken skeletal debris partially fills some original void areas and is also encased in marine cements. In many areas, erect in-situ fenestrate bryozoans coexisted with the microbial hosts. These bryozoans lived in the void spaces between the thrombolitic masses possibly taking advantage of increased current velocity generated by flow between the host masses. Several of these windows display a distinct alignment of bryozoan fronds and long dimensions of the micrite ovals. This orientation is consistent with the orientation of hundreds of articulated crinoid segments measured by Hunt in the surrounding flank and intermound strata (Fig. 2). The similar orientations strongly suggest that the buildup top was swept by regional currents, and probably formed below fair-weather wave base. A core taken by the USGS at the topographic top of Muleshoe recovered 23 meters of the upper buildup strata (Shinn, et al, 1983). Much of this core consists of crinoid-rich skeletal grainstones alternating with thick successions of thrombolitic micrite- and cement-rich mound facies. The presence of skeletal grainstones on the buildup crest suggests the buildup was intermittently exposed to storm energy and that crinoid communities were not confined to Muleshoe's slopes as previously suggested. Instead, crinoid-and fenestrate bryozoan/microbial-dominated communities alternated in response to changing conditions. Present crest exposures at Muleshoe are dominated by the fenestrate bryozoan/microbial facies as crinoid grainstones are more easily eroded.
These mound strata are cut by a number of large neptunian dikes that are again evidence of the buildup's instability as it built up and out over compactible flank and intermound strata. These dikes include some of the largest and most laterally persistent dikes seen in the present Muleshoe exposures. One legendary dike, last seen in 1958, reportedly measured over 20 meters in length. Lloyd Pray nearly lost his life in pursuit of this mythical beast, and would appreciate any updates on its location.
The view to the southeast provides one of the best vantages from which to study the stratal relationships of the intermound and post-Muleshoe units. Buckling of the Hackberry beds is evident. These coherent strata slid off Muleshoe's flanks during compaction of the intermound Table Top and Arcente beds. Cliff exposures to the east are composed of the overlying Dona Ana and Rancheria units. The Dona Ana thickens dramatically and drops in elevation away from Muleshoe. These changes are partially due to differential compaction of the intermound Arcente strata (Hunt and Allsop, 1994; Hunt et al., 1995). Geopetals in the Dona Ana show systematic variations that confirm that the present dip of the Dona Ana package is largely compactional in origin. Compaction of the Arcente created topographic relief at the top of the Dona Ana beds. Dona Ana strata overlying Muleshoe were then preferentially removed by erosion prior to deposition of the Rancheria strata. An angular unconformity sharply truncates the Dona Ana beds and clasts of Dona Ana chert as present in the basal Rancheria strata.
Once you have seen the mound textures and an overview of the southern flank and post mound strata, proceed up to the Dona Ana exposures at the eastern crest of the buildup.
The angular unconformity between the Dona Ana and Rancheria units can be studied in detail at this station. An in-situ Dona Ana mound underlies this surface towards the crest of Muleshoe. These Dona Ana mound strata are remarkably similar to some mound facies in the underlying Muleshoe complex. In a sense, these lower Dona Ana strata can be viewed as another Muleshoe growth phase after a particularly long hiatal period. As compaction of the intermound Arcente strata occurred, Muleshoe's resistant bulk became a topographic high. During Dona Ana deposition this relief was colonized by a carbonate mound community, and the Muleshoe area served as a point source of carbonate production. Prograding clinoforms form a laterally persistent unit that continues north into Arrow Canyon (Hunt et al., 1995).
Proceed to the topographic crest of Muleshoe where a concrete slab marks the location of the USGS core, and then to the north shoulder of Muleshoe for a good view of buckled Hackberry beds that slid down Muleshoe's northern flank. Alternatively, if time is short, proceed directly down the north side of Muleshoe to the next station. Be careful on this descent, the slope is steep and is often covered by loose talus.
Station 18 is the top of a small gully immediately north of Muleshoe. This gully cuts through the northern flank strata equivalent to growth phases V, IV and possibly III. This flank section is nearly the same distance from the center of Muleshoe as the flank section traversed on the south (basinward) side of Muleshoe. Although the two sections share many similarities, these northern flank beds are not as grain-rich as those to the south. Numerous thin bedded, fossiliferous wackestones and mudstones are intercalated with grain flows that originated on Muleshoe's crest. Chert is more abundant on this northern flank, possibly indicating a increased abundance of sponge spicules in these strata (Meyers, 1977).
Base of same gully section. Thick breccias cap the gully's upper Alamogordo section. These beds occupy the same stratigraphic position as the breccia interval that overlies the Alamogordo beds on Muleshoe's south side. Both these breccia intervals appear to have been associated with the hiatal boundary separating growth phases II and III. Articulated segments of crinoid coiumnals are exposed on bedding planes in the overlying flank strata. Such segments are common in lake Valley strata and often exhibit a distinct alignment, presumably reflecting current or slope direction (Fig. 2).
Contour out along the Alamogordo level towards the large exposed outcrop lens northwest of Muleshoe.
Deformed and truncated Alamogordo strata are exposed along the base of this outcrop lens. Their deformation and erosion was associated with the emplacement of a large (several meters deep) channel. The abrupt truncation of host strata and the compound nature of the channel fill is well exposed in the lower west and southwest faces. Many of the channel-filling units contain clasts of mound center and mound margin facies. A thick megabreccia unit fills the top of the channel and drapes the adjacent Alamogordo strata. This megabreccia contains clasts over 10 meters long. Although these blocks are not in-situ, they contain excellent examples of the mound margin fabric. Bryozoans and other skeletal elements were partially silicified and stand out in relief on the naturally-etched surfaces. The growth form of the bryozoan colonies can be examined on exposed surfaces that were originally vertical sections of the mound margin. Individual bryozoan colonies built up from a single point to over a meter in height and cover several square meters of surface.
While its not possible to physically correlate these beds to Muleshoe's south side, this megabreccia occupies a similar stratigraphic position to the thick southern megabreccias that overlie the growth phase 11 I/I V boundary. This northern breccia is also interpreted to reflect collapse of the mound margin during periods of mound hiatus, possibly due to differential compaction of flank and intermound deposits.
Make your way down the slope to the small rise west of Muleshoe and from there back to the starting point. Stop on the rises northwest and southwest of Muleshoe. These are excellent vantage points from which to view the stratal relationships on Muleshoe's northern flank and in the southwestern face.
Many Waulsortian mounds and related micrite mounds are interpreted to have initiated in deep water, below the photic limit (Pray 1982; Lees, Hallet, and Hibo 1985; Lees and Miller 1985; Ausich and Meyer 1990). This interpretation is based on their toe-of slope paleo-geographic position, inferred mud component, and the absence of grain micritization or a recognized green algal community (see Lees Hallet, and Hibo 1985, Lees and Miller 1985; for discussion). Muleshoe Mound initially appears to fit these criteria, yet detailed field and petrographic study reveal characteristics that may discredit a deep, quiet water origin.
Much of the mound micrite is microbial in origin, hence quiet water was not necessary for its accumulation. Textures of the microbial (thrombolitic) fabric vary systematically through the buildup, correlating with independent evidence of depositional energy such as winnowing, abrasion, intraclasts, current-aligned skeletal components and bryozoan taphonomy. Periods of higher energy are indicated by scour and channel features, rounded lithoclasts of submarine-cemented core facies, and coarse grainstones that filled dikes and reef mound cavities.
Micrite precipitation occurred primarily within a soft-bodied host, although it extended into pore spaces between crinoid grains that buried the hosts. We have not yet been able to identify the host organism, but suspect that Muleshoe originated as a dominantly algal buildup. It should be emphasized that not all micrite is precipitated. Bioturbated, deposited mud occurs downdip of the mound core facies along Muleshoe's slopes and in the surrounding off-mound strata. In the upper growth phases that have significant topographic relief, many of the features we've interpreted as evidence of higher energy (cements, intraclasts, erosion, alignment, channels, etc.) are generally restricted to upslope areas of mound core growth. The overall pattern of facies in Muleshoe can be viewed as a transition from relatively simple, undifferentiated mounds growth phases (I and II) to well-differentiated complex growth phases as buildup topography increased and the mound spanned a greater spectrum of environments.
Internal stratal patterns suggest that Muleshoe did not originate at great depths (100's of meters). Initial growth at Muleshoe was aggradational, but as the buildup gained relief, there were pronounced shifts to first symmetrical and later asymmetric progradation. Systematic variations in the height and stratal patterns of other Lake Valley buildups are consistent with mound growth occurring under a regional accommodation level. Graded crinoid beds alternate with micrite- and cement-rich mound facies in a core taken from the top of Muleshoe. These are here interpreted to be evidence that Muleshoe experienced intermittent storm activity. If correct, the regional accommodation level for mound growth must have been above maximum storm base (and may have been as shallow as fair-weather wave base). Presumably this would place Muleshoe well within the photic zone, as there is little evidence of turbidity. The general lack of micritized grains may reflect environmental controls other than depth. Pickard (1993) reports a paucity of micritized grains in Scottish Waulsortian mounds, even though micritized grains are relatively abundant in the correlative off-mound deposits. This distribution suggests a non-depth related control on micritization.
Early Carboniferous Ocean System
Our proposed setting for the Lake Valley mounds is similar to that of many modern reefs, yet the mounds themselves are quite different from modern framework reefs. The authors suspect that many of the 'enigmatic' characteristics of these buildups are indications of fundamental changes in the evolution of the ocean system. Nutrient levels, oxygen levels, or cooler temperatures, individually or linked, may have all played a role.
Oxygen stress was certainly a significant, if intermittent, factor in Lake Valley basin deposition. Although the upper growth phases (ll-V) at Muleshoe contain an abundant diverse fauna, the correlative basin section is composed of hiatal surfaces and black, laminated lime mudstones (Table Top Member) that contain sparse fauna and occasional Zoophycus and Chondrites burrows - an association characteristic of oxygen depletion (Ekdale and Manson 1988). Bioturbated intervals that contain a mixed fauna of corals, bryozoans, brachiopods and crinoids are interspersed though this dominantly oxygen-depleted section, reflecting ephemeral periods of aerobic bottom waters. A pattern of episodic oxygen depletion also exists in many other Lower Carboniferous basins (Kirkby 1994; Lineback 1985; Smith 1972, 1982). These basin-fills typically consist of vertical alternations of thick sections of laminated (anaerobic) strata and thin intervals of burrowed, sparsely fossiliferous (dysaerobic to aerobic) strata. Periods of ocean anoxia occurred during the Late Devonian and Devonian/Carboniferous intervals (Klemme and Ulmishek 1991; Savoy 1992, Joachimski and Buggisch 1993). The common occurence of dysaerobic to anaerobic sediments in many Waulsortian basins suggest a tendency to ocean anoxia may have persisted through the Tournasian.
Nutrient and oxygenation levels would most likely be closely linked. Higher nutrient levels would tend to promote anoxic layers, and anoxia in turn preserves higher nutrient levels. In modern carbonate buildups, increased nutrients results in algal 'fouling' of the reef surface, a consequent inability of corals to successfully propagate, and demise of the reef framework (Wood 1993). Wright (1994) has previously pointed out the potential importance of nutrient upwelling on Waulsortian mound development, and Wood (1993) suggested that Waulsortian mounds may reflect Early Carboniferous oceans having had higher nutrient levels and greater upwelling rates than the modern ocean system. While we concur that the Early Carboniferous seas may have been more nutrient rich, proving nutrient levels is a difficult matter. In this study, bioturbation was used as a proxy for oxygen levels and only as a possible indicator of nutrient levels.
However, an upwelling origin for nutrients seems difficult to envision, considering the common, geographically widespread, occurrence of Waulsortian mounds associated with anoxic basin waters. Waulsortian mounds in the Lake Valley and Pekisko formations grew on very gentle ramp slopes that appear to have been an order of magnitude less steep than modern upwelling areas. Further, if present reconstructions of the Lake Valley and Pekisko basins are correct (Hunt, 1994; Kirkby and Simo, 1993; Hunt and Kirkby, 1996), these basins had limited, if any access, to abyssal water layers. We suggest it is more likely that Early Carboniferous shallow seas adjacent to continental masses had consistently higher nutrient inputs due to increases and changes in the terrestrial plant community. Models of catastrophic ocean nutrient levels have been proposed as a forcing mechanism for the Frasnian/Fammenian extinction. Continued higher nutrient levels may have been responsible for the relative stability of abundant Waulsortian buildups throughout the earliest Carboniferous.
Muleshoe is composite Waulsortian buildup that grew episodically. On the basis of stratal patterns, erosion surfaces, internal facies tracts, and diagenetic features, the buildup can be divided into five distinct growth phases that represent separate recolonizations of the mound topography. Muleshoe intercalates and is encased by basin strata composed of anaerobic-to-dysaerobic mudstones alternating with thin intervals of well-oxygenated fossiliferous wackestones. The buildups' internal architecture and relation to oxygen deficient strata comprise previously unrecognized responses of the mound to environmental fluctuations.
Serial slabbing of large mound blocks confirmed a microbial role in the genesis of this previously enigmatic buildup. Microbial precipitation within an organic precursor, combined with extensive submarine cementation created a mound framework rigid enough to create depositional relief and shelter large voids. Textures of the microbial fabric vary systematically through Muleshoe, correlating with independent evidence of depositional energy such as winnowing, abrasion, intraclasts, current-aligned skeletal components and bryozoan taphonomy. A relatively shallow-water origin (<100 meters) is also supported by the change in depositional style from aggradational to progradational stratal patterns. This trend is unlikely for a mound previously interpreted as deep (subphotic) water buildup. Regional patterns of correlative Lake Valley strata confirm that Muleshoe grew at a depth where accommodation space was a primary control on mound geometry. Muleshoe's episodic growth, microbial origin, extensive marine cementation, and close association with anaerobic-to-dysaerobic basin strata suggests an intriguing potential correlation between mound growth and pronounced stratification of the Early Carboniferous ocean.
In this present revision, Muleshoe Mound provides yet another, hopefully improved, model of Waulsortian mound growth. Many features seen at Muleshoe occur in other Waulsortian mounds suites such in the Pekisko Formation of Alberta (Kirkby, 1994), the Lodgepole Formation of Montana and even in much smaller mounds in the Fort Payne Limestone of Kentucky. These shared characteristics (episodic growth, microbial fabrics, energy indicators and an association with anoxic basin strata) provide a important step forward in our understanding of these buildups and the Early Carboniferous ocean system. However, a Muleshoe model shouldn't be overplayed. For all their similarities, each of the suites exhibits individual characteristics and marked differences occur within the suites that are difficult to discern with limited core and outcrop data. These differences are as important as the shared characters in understanding the range of these remarkable buildups. Again, our understanding of Muleshoe is a work-in-progress, and we would be very disappointed if it remains unchallenged.
The crest of Muleshoe mound affords some striking views of the Dona Ana (Figs 20-22). The lower M1 mounds are highly asymmetric and have steep basin facing and migrating slopes (Fig. 22). These geometries are best developed in Arrow Canyon, and are similar to those developed within the Danish Chalk (e.g. Surlyk, 1997). In Figure 22 the basinward progradation of the lower mound suit is evident. Strike sections, in the SE corner of Muleshoe Canyon, show these mounds to be symmetrical and domal, with width:height ratios of 2:1-10:1. The lower mounds are interpreted to be migrating into a northerly-directed paleocurrent (following Surlyk, 1997).
In contrast, the M2 mounds show both northerly and southerly directed clinoform progradation. They radiate away from Muleshoe mound, and are intercalated with erosionally-based graded beds (e.g. Fig. 20C). In strike section they are mound shaped and show bilateral downlap (Figs 19, 20). Both M1 and M2 mound suites downlap onto condensed sections, locally containing glauconite. Both the M1 and M2 mound suites are also erosionally truncated below the lithoclastic strata (e.g. Fig. 1). Whereas their lower surfaces are considered to be condensed sections formed in response to base-level rise, their upper surfaces are interpreted to have formed during base-level falls and lowstands. Thus, the mound suites are interpreted as relative highstand deposits, and the overlying pack and grainstones to represent falling baselevel and lowstand deposits. The base of the Dona is therefore interpreted as a sequence boundary formed in response to base-level fall.
This study was funded by grants from the Petroleum Research Fund, administered by the American Chemical Society, and Union Pacific Resources (to Kirkby) and the Natural Environment Research Council of Britain (fellowship to Hunt); their support is greatly appreciated. Additional funding came from the American Association of Petroleum Geologists, Geological Society of America, University of Wisconsin - Madison, Amoco, Arco, Unocal, Exxon Production Research, and Sigma Xi. The study benefited greatly from the collaboration and suggestions of Lloyd Pray, Coco van den Bergh, Wayne Ahr, Bob Stanton, Dave Jeffery, Tim Allsop, and Steve Bachtel - although the authors are solely responsible for all heresies.
Lloyd Pray deserves special acknowledgment. The fieldguide approach used here was based on Lloyd's original Muleshoe road log (Lane et al., 1982). By integrating text to field photos, Lloyd made it easy to identify specific locations and features. When we began our work at Muleshoe, we quickly found that Lloyd's approach made his work the most useful published Muleshoe study. By adopting his style, we hope to make it easy to find exposures and features discussed, regardless of whether you agree or disagree with our interpretation. Finally, special thanks to Du Anne, Liz and Eva for not fleeing when they had the opportunity.
Figures & Tables
The Carboniferous Geology of the Sacramento Mountains New Mexico
This field guide was originally put together for the SEPM Research Conference on Permo-Carboniferous Carbonate Platforms and Reefs in 2000 (SEPM SP 78 contains the research papers from the conference) and was only distributed to those attending the conference. After receiving copyright permissions, it has become an official publication of SEPM - Field Guide No. 12 and is available on CD. This guide begins in the Dry Canyon area of the Sacramento Mts. devoted to platform evolution during Missourian and Virgilian times, focusing on mounds and cycles of the Holder Fm. Also covered is the Mississippian Lake Valley Formation's sedimentology and depositional environments in Indian Wells Canyon and Alamo Canyon again with focus on mound biota and comparative sedimentology of mound versus level-bottom biota. Muleshoe Mound in the southern part of the Sacramento Mts. is used to examine mound growth history and an example of differential compaction and modern day appearance of outcrops. This volume includes background geology on all of the areas that are visited as well as detailed discussions about each of the field trip stops.