Diagenetic alteration of impact spherules in the Neoarchean Monteville layer, South Africa
Published:January 01, 2006
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Issaku Kohl, Bruce M. Simonson, Melissa Berke, 2006. "Diagenetic alteration of impact spherules in the Neoarchean Monteville layer, South Africa", Processes on the Early Earth, Wolf Uwe Reimold, Roger L. Gibson
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Intercontinental correlation of distal Archean impact ejecta layers can be used to help create a global time-stratigraphic framework for early Earth events. For example, an impact spherule layer in the Neoarchean Monteville Formation (Griqualand West Basin, South Africa) may be correlated with layers in one or more formations in Western Australia. To help assess the degree to which diagenetic alteration would hinder such correlations, we performed a petrographic study of spherules in the Monteville layer. Most of the spherules in the Monteville layer have botryoidal rims composed of radial-fibrous K-feldspar, but compaction and replacement have greatly altered their appearance and mineralogy. Moreover, the Monteville spherule layer consists of three main subunits, and spherule compaction varies between subunits as well as across the Griqualand West region. Compaction is about three times greater in a medial spherule-rich subunit as compared to a basal subunit rich in large intraclasts, resulting in better preservation of the shapes of melt particles in the latter. However, spherule rims have comparable numbers of fractures in both subunits, indicating the melt particles were fractured prior to compaction. Some spherules contain mica ribbons with a septarian geometry. Fracturing via rapid thermal quenching could help explain all of these features. If hot spherules possessing crystalline rims were thermally shocked when they hit the ocean, fractures would have the observed geometries and provide pathways for fluid infiltration and local replacement of glass by mica. Although heavily distorted, impact spherules in the Monteville layer are very similar to those in the Hesta occurrence of the Neoarchean Jeerinah spherule layer of the Hamersley Basin, even showing similar diagenetic histories. In this instance, diagenetic alteration may actually help rather than hinder intercontinental correlation of impact spherule layers.
Asteroid impacts were instrumental in shaping the early Earth's environment. These powerful events are recorded in the craters that they leave behind, as well as their proximal and distal ejecta deposits. Oceans cover most of the current Earth's surface and probably covered an even larger proportion of it early in Earth's history (Eriksson, 1995). This suggests that a majority of these early impacts occurred in the open ocean or on continental shelves instead of on dry land. However, due to the lack of pre-Mesozoic oceanic crust, workers can only make inferences from distal ejecta deposits that date back to 3.47 Ga (Byerly et al., 2002). Distal ejecta layers also have the potential to serve as important stratigraphic markers due to their relatively instantaneous emplacement and regional or even global distribution (e.g., Montanari and Koeberl, 2000; Koeberl and Martinez-Ruiz, 2003). In order for these layers to be effective time-stratigraphic markers, workers must be able to correlate distal ejecta deposits reliably on an intercontinental scale. This has proven relatively effective in the case of the K/T impact (e.g., Smit, 1999), but Archean successions lack the rich fossil record that made this global correlation possible. Strong diage-netic overprinting is also a potential impediment hindering the correlation of spherule deposits in different geographic areas.
The goal of this study was to investigate the textures of spherules from a Neoarchean impact layer in the Monteville Formation of the Griqualand West Basin, South Africa, which is potentially contemporaneous with late Archean layers found in the Hamersley Basin, Western Australia (Simonson et al., 1999, 2000a), to see whether diagenetic alteration would render this correlation unworkable. Although there is clear textural evidence that the spherule population in the Monteville layer was seriously modified by diagenetic processes, especially via compaction and replacement, we found parallels between these spherules and those of certain layers in the Hamersley Basin that support the proposed correlation. We hope our results will assist future workers as new layers are discovered and new correlations are attempted.
The Monteville Formation is the lowermost unit of the Campbellrand Subgroup of the Transvaal Supergroup in the Griqualand West Basin, South Africa (Fig. 1). The Campbell-rand Subgroup is a major carbonate buildup located along the western margin of the Kaapvaal Craton. Most of the preserved Campbellrand underlies the Ghaap Plateau, which occupies a large part of the Griqualand West Basin (Beukes, 1987). The Monteville Formation is best exposed on the northeastern edge of the basin along the Campbellrand escarpment, but it extends throughout most of the basin in the subsurface. Stratigraphically, the Monteville Formation occupies a transitional position between the underlying shale-rich Lokammona Formation, which was deposited in relatively deep water, and the overlying Reivilo Formation consisting of shallow-water carbonate deposited on a platform directly above the Monteville Formation. The lithologies and sedimentary structures of the Monteville Formation make it clear that it contains the last deep-basinal strata deposited in a deeper shelf to slope environment before the transition to an extended period of platformal deposition known as the Ghaap Facies (Beukes, 1987).
The Monteville Formation contains a single layer rich in impact spherules that has been identified in three distinct geographic regions (Fig. 1; Simonson et al., 1999). Isotopic age dates of tuffs located within the Transvaal Supergroup constrain the spherule layer's age between 2640 Ma and 2520 Ma (Simonson et al., 2000a). Known surface exposures of the spherule layer are restricted to the Campbellrand escarpment, the best ones occurring on the Monteville Farm at the type section of the formation. Here we define the Monteville Farm region as including the Monteville Farm site proper (MF) and two closely spaced sites 50 km to the southwest of Monteville Farm at locations informally designated Reid's Drift (RD) and Goudkop (GK) (Fig. 1). Reid's Drift and Goudkop were not mentioned in Simonson et al. (1999) because they were discovered only after this paper was published. The Monteville spherule layer was also intersected in a suite of drill cores (RF1, V1, TK1, P, LB1, K1, TL1, SF1) in a trapezoidal region centered on the recently closed Pering Mine 145 km north-northeast of Monteville Farm (Fig. 1). Samples from most but not all of these cores were studied in detail. The Monteville spherule layer was also intersected in a third distinct area in the deep subsurface by a single research core drilled near Kathu, which is located 130 km west along the edge of the Griqualand West Basin (Fig. 1). We did not include samples from the Kathu core in our study because spherules and related particles are present in low concentrations and have been extensively replaced by carbonates.
Monteville Farm Region
In the Monteville Farm region, the Monteville layer's measured thickness is 29–80 cm, and it is divided into two distinct subunits (Simonson et al., 1999). The lower subunit of the spherule layer consists of gravel-size intraclasts mixed with well-sorted, sand-size interstitial material. The intraclasts consist of dolomite, shale, and pyrite and have textures suggesting they were derived from the underlying strata (Simonson et al., 1999). There is a wide size range within the intraclasts and, like all Monteville strata in surface exposures, they show obvious signs of recent surface weathering, including pyrite oxidation. Carbonate intraclasts can be as large as 180 cm long and 13 cm thick, and shale intraclasts are up to 13 × 18 cm in cross section. Pyrite intraclasts are rarely longer than a few centimeters (Fig. 2).
The interstitial sand ranges down to fine sand size and is composed in part of the same constituents as the intraclasts, but there is also an abundance of spherules. The spherules and other interstitial sand are concentrated in discrete pockets, which we interpret as large pores between intraclasts (Fig. 2). The intraclast-rich subunit is overlain by an upper subunit that is much finer and less spherule rich, consisting mostly of fine- to medium-sand–size carbonate detritus, with sparse gravel-size intraclasts. The carbonate is coarsely crystalline diagenetic dolomite, and the few spherules present exhibit textures similar to those contained in the lower subunit of the layer, although they are commonly fragmented. Although detrital textures are largely obscured, the presence of probable hummocky cross-stratification indicates that the upper subunit originally consisted of well-sorted sand (Simonson et al., 1999; Hassler and Simonson, 2001). We did not apply our techniques to samples from the upper subunit in the Monteville Farm region, because spherules and related particles occur in relatively low concentrations and are extensively replaced. In addition, we were only able to identify the upper subunit at Monteville Farm but neither at Reid's Drift nor at Goudkop, and we were unable to identify the medial spherule-rich subunit described below at any of the surface exposures (Fig. 3).
Pering Mine Region
The Monteville spherule layer was identified in nine cores drilled in a trapezoidal area 65 km north to south, 20 km east to west, and centered around Pering Mine (Simonson et al., 1999) (“P” in Fig. 1). In most of these cores, the lowest subunit of the layer contains a high concentration of spherules in an ∼5-cm-thick bed that possesses a sharp erosional base. Again in most cores, this spherule-rich basal subunit is overlain by a distinctly different subunit that contains a much lower abundance of spherules. This upper subunit consists of a thin layer of quartzose sandstone in the southernmost cores but expands to a 2-m-thick layer rich in carbonate in the northernmost cores (Fig. 5 in Simonson et al., 1999). Spherules tend to be finer, more fragmented, and widely dispersed in this upper subunit, which we correlate with the upper subunit at Monteville Farm (Fig. 3). Small, unaltered pyrite intraclasts are found in these cores but are much finer than their counterparts at the Monteville Farm locality. In addition to these two subunits, the SF1 core from the southern edge of the Pering Mine Region (Fig. 1) contains a basal subunit beneath the spherule-rich subunit. The basal subunit is rich in gravel-size intraclasts of carbonate and pyrite that are similar in size and abundance to these phases in the lower subunit of the layer at Monteville Farm (Simonson et al., 1999). The basal intraclast-rich subunit is ∼50 cm thick, and the material between the intraclasts consists of fine- to medium-sand–size dolomite detritus, with high concentrations of spherules (Fig. 2). We did not apply our techniques to samples from the upper subunit in the cores of the Pering Mine region, again because spherules and related particles occur in relatively low concentrations.
Samples were first screened by general petrographic examination, including comparison to published photomicrographs and descriptions of impact spherules from various layers including the K/T boundary layer (Smit et al., 1992; Smit, 1999), late Eocene microtektite and microkrystite strewn fields (Glass, 2002), other microtektite strewn fields (Simonson and Glass, 2004, and references therein), and late Archean layers in the Hamersley Basin of Western Australia (Simonson et al., 1999, 2000a, 2000b; Simonson, 2003). We also examined spherule-bearing thin sections from the K/T boundary layer at the Mimbral site in northern Mexico (Smit et al., 1992) and ODP (Ocean Drilling Program) site 1049C drilled on Blake Nose (Norris et al., 1999) for comparison. Eight samples from three surface sites in the Monteville Farm region and ten samples from nine cores in the Pering Mine region that were rich in well-preserved spherules were then chosen for quantification of the characteristics of the spherules via the methods described in the following paragraphs.
Over 400 points were counted for each of 18 samples to determine the relative abundances of 8 textural categories (Table 1) encompassing all the former melt particles in the Monteville layer. The relative abundances of these categories were then tabulated independent of the presence of other types of detritus (Table 2). In addition to spherule types, the abundance of interstitial material, fine-grained sheet silicates, sparry carbonate crystals, and particles with internal textures completely obscured by replacement were also quantified by point counting. The replacement noted in surface samples was mainly by carbonate and pyrite (the latter weathered subsequently to iron oxyhydroxides). In core samples, replacement is dominated by sheet silicate phases, most likely sericite and a chlorite phase, as well as minor amounts of carbonate in certain samples. None of the carbonate minerals were analyzed quantitatively, but the majority appear to be dolomite rather than calcite based on their weak response to dilute HCl.
Average aspect ratios (long dimension/short dimension) of spherules were determined by measuring long and short axis lengths for 35–40 former melt particles with botryoidal rims (categories 1, 2, 4, and 6) in each sample. Restricted areas judged to be typical were chosen in a given sample, and then all spherules in that area were measured. The thickness and number of fractures showing offsets in the rim of each particle were measured at the same time (Table 3). The average number of contacts per grain was determined by counting the actual number of contacts in each of 20 successive grains along a horizontal transect within a given sample in an area chosen to be typical of each core or surface exposure.
In addition, electron microprobe analyses of a typical spherule from sample SF1–1 were obtained (Table 4). The spherule was in a polished thin section that was carbon-coated prior to analysis. The electron beam was set for all analyses at 15 kV acceleration voltage and 10 nA absorbed beam current on brass. Spectra for each spot analysis were acquired for 80 seconds. Analyses were performed at Rand Afrikaans University (now called University of Johannesburg) on a Cameca CAMEBAX 355 electron microprobe system equipped with 3 wavelength-dispersive spectrometers (WDS) and an additional Link energy-dispersive spectrometer (EDS), carrying a Li-drifted Si-detector. The system was controlled by the Link ExL II and Lemas software packages. Only the EDS system was utilized for the analyses. Routine calibration of the energy-dispersive spectrometer was performed using a pure Co metal standard. The raw EDS spectra were automatically ZAF-corrected by the ExL II software.
Monteville Farm Region
The eight samples from the spherule layer at all three sites in the Monteville Farm region are fairly uniform in composition. Their major constituents are gravel-size shale and pyrite intraclasts mixed with fine to very coarse sand consisting of a mixture of spherules, irregular melt particles, carbonate grains, and shale and pyrite fragments. The shale fragments possess tabular to angular shapes with occasional relict bedding. The large intraclasts appear to form a grain-supported framework, indicated by the local presence of sparry carbonate fillings in what appear to be sheltered pores above geopetally distributed carbonate sand and spherules. Original clast outlines within the sand fraction are often obscure due to replacement by coarse carbonate crystals (Fig. 4). Sparry carbonate crystals between intraclasts and former melt particles probably represent void-filling cement and/or replacements of earlier generation cement. Other carbonate crystals occur in pyrite-free, cross-cutting veins that can be up to 2 mm wide. Some of the vein crystals show deformation twins indicative of temperatures as high as 300 °C (Passchier and Trouw, 1996). These veins cut across a petrographically distinct type of carbonate consisting of crystals 50–100 μm large, with intercrystalline pyrite.
All of the samples from the Monteville Farm region are from surface exposures and show extensive evidence of weathering, most prominently in the form of iron-oxide staining. The sources of this iron are most likely the pyrite intraclasts (which are almost entirely oxidized) and possibly ferroan carbonate phases. In reflected light, silvery metallic cores are visible within red intraclasts; they are thought to be pyrite weathered to varying degrees, whereby the earthy red material is the most altered phase and possibly consists of oxy-hydroxides. The samples also show signs of minor compaction, such as in situ grain fracturing and pressure-solved grain contacts. A selvage composed of sericite, chlorite, and possibly pyrite is sometimes present along elongated grain contacts. Sand grains in the samples we studied from the Monteville Farm region, all from the lower subunit of the Monteville layer, average 1.3 contacts per grain, indicating that they have not been compacted very much. Uncompacted sands consisting of equidimensional, well-sorted clasts average 0.9–1.6 contacts per grain (Pettijohn, 1975), and the sand in the lower Monteville subunit appears to have originally consisted of well-sorted clasts with low aspect ratios (Table 3).
Petrography of Impact Ejecta
Spherical splash forms . These particles show a range of internal textures, with much of this variation reflecting different degrees of replacement. Some are composed of K-feldspar with sericite and, much less commonly, quartz, whereas others have been completely replaced by carbonate save for thin opaque rims with relict, inward-radiating, botryoidal terminations (Fig. 4). Different amounts of sparry carbonate crystals are present within some of these particles. Still other spherules show indiscriminant replacement by large angular crystals of an opaque phase, most likely hematite formed from authigenic, now surface-weathered pyrite.
Although textures within the spherules that are not obscured by replacement are highly variable, one or both of two textural elements appear, to some degree, in almost every spherule, as is typical for late Archean to early Paleoproterozoic impact spherules in general (Simonson, 2003). Acicular to lath-shaped crystals of what is now K-feldspar constitute one of these elements. Most of the K-feldspar crystals are arranged in fan-shaped radial arrays. Much less commonly, spherules contain lath-shaped crystals that are randomly oriented. A complete continuum exists between spherules in which the K-feldspar laths form rims and those in which the laths fill the entire cross section; this latter texture will be referred to hereafter as hololathic.
The other common textural element involves clear internal spots that are highly variable and filled by sheet silicate, quartz, K-feldspar, and/or carbonate. Two optically distinct varieties of sheet silicates are present, one appearing clear and the other pale green in plane polarized light. The clear sheet silicate is most likely sericite, whereas the pale green one is composed of smaller crystals and appears to be chlorite, as it exhibits lower birefringence than the sericite (although this could be an impression caused by the smaller grain size). Some of the clear spots possess well-defined circular to oval cross sections; rare teardrop-shaped clear spots are also present. We interpret these as filled vesicles within melt droplets by analogy to similar textures in Phanerozoic impact spherules, especially those found in the K/T boundary layer (see Fig. 5 in Smit et al., 1992; Smit, 1999). In contrast to the clear spots with well-defined geometric shapes in cross section, other clear spots have more irregular boundaries defined by the uneven termination of inwardly radiating K-feldspar (see Fig. 7b in Simonson, 2003). These are more likely to have originated as relict glass cores that were dissolved and/or replaced during later diagenesis and are again analogous to observations of certain K/T spherules (Bohor and Glass, 1995). Finally, many spherules have clear central spots that are so extensive that their origin is unclear. Quite a few spherules have been so extensively replaced by carbonate that all internal textures have been obliterated and all that remains are thin opaque botryoidal rims. Replacement carbonate occurs as microcrystalline to blocky crystals, the latter filling spherules with only one to three crystals. It is possible that bubbles actually occupied most of the interiors of some of these spherules.
Nonspherical splash forms . A minority of the particles with spherule-type internal textures have shapes that are highly elongate and similar to those of peanuts, tear drops, and other more exotic splash forms. Some have thin opaque botryoidal rims and are completely replaced by blocky carbonate crystals. Such particles are often oriented perpendicular or oblique—instead of parallel—to bedding, indicating they were not produced by the compaction of spheres. Some elongate splash forms exhibit hololathic textures. K-feldspar crystals within these nonspherical particles tend to be at the larger end of the K-feldspar size spectrum, resulting in relatively more coarsely crystalline particles.
Fragmented splash forms . Fragments of broken spherules and other particles are abundant and have two distinct modes of occurrence. The most easily recognizable fragments exhibit radial-fibrous or botryoidal textures typical of spherule rims, and some occur as isolated fragments, whereas others occur in clusters. Most isolated fragments consist of K-feldspar arranged in radial fibrous fans with occasional opaque rims exhibiting botryoidal terminations on one side versus smoothly curved circular boundaries that represent arcs of a larger circle on the opposite side. This indicates that these particles are segments of a once-continuous rim, and the fact that the individual fragments do not resemble or fit into any clasts in close proximity suggests that they were fragmented prior to deposition. The fragments range in character from “pie slices,” with just one or two adjacent fans on the former rim, to more extensive rim fragments that represent up to half the circumference of an original spherule.
In contrast to isolated occurrences, other fragments occur in clusters of slightly curved to angular fragments that closely resemble one another and in many cases clearly fit into one another. We observed two types of clusters: (1) groups of randomly oriented fragments, where rims appear to have been completely disarticulated and displaced (Fig. 5; lower portion), and (2) groups of particles displaying ovoid cross sections (Fig. 5; central grain). Some of the most elongate clusters show little displacement of the broken pieces and preserve, to a degree, a continuous rim (Fig. 6). In the ovoid clusters, the botryoidal sides of the fragments generally face inward, whereas the smoother, curved edges face outward. Both elongated individual fragments and the long axes of the clusters themselves are usually parallel to bedding. In intraclast-rich samples, fragment clusters often contain sericite internal to the broken rim, although carbonate usually occupies most of the area inside the rim. Some clusters define a circular outline that is nearly undisturbed save for thin carbonate-filled gaps that cut the rims and are optically continuous with the carbonate crystals on one or both sides of the rim (Fig. 4). The fragments in circular clusters are geometrically similar to sectors of intact spherules with botryoidal rims.
Irregular particles . The Monteville layer in the Monteville Farm region also contains irregular particles of former melt, textures of which are almost identical to those in the Carawine layer of the Hamersley Basin (Simonson et al., 2000b). Irregular particles are generally larger and more angular than the spherules and show an array of internal textures. Almost all of the large irregular particles have two distinct textural aspects. The first consists of light bands composed of K-feldspar alternating with dark to light bands of sheet silicate. The bands form patterns that range from planar to wavy to completely swirled and strongly resemble flow bands in molten materials such as tektites and obsidian. Infilled bubbles are the second major textural element. They range in abundance from 0% to around 20% by area and are filled by sericite of varying coarseness. They are similar to vesicles in the spherules, although they tend to be smaller. Even particles that were extensively replaced by sheet silicate are still identifiable as “irregulars” by their abundant vesicles.
Mica ribbons . In six of the surface samples, a small percentage of spherules contain one or more isopachous sericite ribbons that appear to consist of fibers, as they display coherent length–slow extinction (Fig. 7). The ribbons occur in three distinct textural positions: (1) coating botryoidal terminations along the inner edges of rims, (2) lining the edges of vesicles in a concentric orientation, and (3) forming a radially branching network throughout the irregular (i.e., nonvesicular) clear spots. The ribbons forming branching networks resemble the “mesh texture” of serpentinites (e.g., Fig. 5.99 in Vernon, 2004) in that original material appears to have been replaced by parallel fibers nucleated on and growing away from a network of microfractures. Where ribbons occupy two or more of the three textural positions mentioned above within a given spherule, they are connected to form a network that appears to have a septarian geometry (Fig. 7). The remainder of the material filling clear spots in spherules containing mica ribbons consists of either radial fibrous K-feldspar or coarser crystalline muscovite. Ribbons with the same geometries that lack coherent extinction patterns also occur in some spherules. These ribbons are composed of the chloritic sheet silicate instead of sericite, tend to be slightly wider, and have a massive texture. A given spherule may contain ribbons with either one or both compositions, but no sericite ribbons were seen in spherules that are dominantly chloritic. In spherules with chloritic ribbons, clear spots are filled with relatively coarser muscovite with or without the presence of any other chloritic sheet silicate. Spherules from the K/T boundary layer at both ODP site 1049 (Fig. 8; see Norris et al., 1999, for location information) and DSDP (Deep Sea Drilling Project) hole 603B (see Fig. 6 in Smit et al., 1992) show textures that appear quite similar to these mica ribbons.
Pering Mine Region, Spherule-Rich Subunit
Spherules from the nine cores in the Pering Mine region average 0.65 mm in diameter (Simonson et al., 1999) and consist almost entirely of K-feldspar and sericite. The space interstitial to spherules and other sand grains is almost entirely occupied by the optically chloritic sheet silicate (Fig. 5). Its uniform appearance and optical orientation suggest that it is authigenic in nature, but it could represent a fine-grained detrital matrix that was replaced and/or oriented by compaction. Minor detrital quartz is also present (Fig. 6), as are opaque minerals that are evenly dispersed and probably authigenic. The fine sizes of the interstitial sheet silicate and opaque fractions support an authigenic origin, as it is highly unlikely that silt- and mud-size particles would be deposited together with well-sorted coarse sand-size particles. In addition, coarsely crystalline carbonate has indiscriminately replaced former melt particles in three samples, leaving traces of melt-derived textures.
There is clear textural evidence that the spherule-rich subunit experienced a high degree of compaction with individual grains having been deformed via both plastic and brittle mechanisms. One line of evidence is the number of contacts per grain, 7.1 on average. An average of 5.2 contacts per grain is typical for well-sorted sands buried to a depth of 2570 m (Pettijohn, 1975). Strong compaction is also evidenced by the fact that almost all grains are in full contact with surrounding grains along their entire perimeters. This is consistent with a high abundance of long and concavoconvex contacts between grains (Fig. 6). Thin selvages of sheet silicates dominated by muscovite but also containing the chloritic phase are found along many of these contacts. Detrital quartz grains appear to have acted as battering rams that indent into former melt particles along concavoconvex grain boundaries (Fig. 6). Since quartz is normally more susceptible to pressure solution than feldspar (Trurnit, 1968), we attribute this behavior to the presence of a thin selvage of insoluble sheet silicate armoring the surfaces of the quartz grains. Simonson et al. (1999) observed similar textures within the overlying quartz-sand–rich subunit, where pressure solution between quartz grains is also evident.
Petrography of Impact Ejecta
Spherical splash forms . Almost every former melt particle in the Pering Mine region shows the same two textural elements noted for the spherules in samples from the Monteville region, namely acicular to lath-shaped crystals of K-feldspar and the presence of circular to irregular, clear, central spots. Commonly radial fibrous aggregates of K-feldspar possess a multitude of submicroscopic inclusions, appearing tan in plane-polarized light, and generally are most abundant near the outer edge of the spherule (Figs. 5 and 6). Acicular crystals vary in size; their widths range from submicroscopic to 10 μm and their lengths from 20 to 200 μm. The laths also vary in size, but they are thicker, ranging from 10 to 20 μm in width, and they are 20–300 μm long. These sizes are comparable to those found in samples from the Monteville Farm region. Aggregates of acicular crystals and/or laths may be arranged to form a rim composed of inward-facing botryoidal terminations or, less commonly when no clear core is present, are hololathic.
As in the Monteville Farm area, the other major textural elements in spherules are clear spots composed of K-feldspar, sericite, and/or a chloritic sheet silicate, with either well-defined circular to oval cross sections or irregular boundaries. Sericite displays one of several habits, appearing in little fans, as vermiform crystal packets, or as larger crystals up to 20 μm long. The habits of the microcrystalline chloritic sheet silicate are similar to the clear sericite, yet individual crystals tend to be smaller. The spots with regular boundaries are often defined by a circle of opaque dust or a band of fine white mica. The irregularedged clear spots begin where the radial fibrous or lath-shaped K-feldspar crystals terminate and are usually composed of sericite, although a small number consist of K-feldspar. Clear spots with irregular edges have been interpreted as replaced glass cores, in part because filled vesicles generally possess circular or ovoid cross sections (Simonson, 2003). Botryoidal textures have been replaced by sericite in some spherules, yet they are still recognizable due to a thin opaque band preserving the telltale scalloped shape of the final growth surface within the sericite (Fig. 9). There are also intermediate stages of replacement where sericite pods can be seen on K-feldspar fans (Fig. 10).
Nonspherical splash forms . This type of particle is rare in the Pering Mine region, but a few teardrop- and peanut-shaped splash forms have been observed. When they are present, they almost always show signs of brittle deformation acting upon the botryoidal rim of the particles. Nonspherical splash forms display both clear spots and hololathic K-feldspar textures internally.
Fragmented splash forms . Just as in the Monteville Farm region, spherule fragments occur both as solitary clasts and in clusters. The isolated fragments are compositionally similar to those found in the Monteville Farm region and, as in those samples, appear to have cracked and to have been separated prior to deposition. The fragment clusters likewise appear to be similar to those in the Monteville Farm region in that we observed two types: (1) randomly oriented fragment collections where rims appear to have been completely disarticulated, and (2) highly elongate clusters in which fragments still line up to define the shape of a once continuous rim. Internally, when a rim can be defined, clusters can be filled with either pure sericite or a combination of sericite and finer-grained chloritic sheet silicate, with minor pyrite (Fig. 11). The sheet silicates within the clear spots in some spherules with relatively continuous rims show a preferred length–slow orientation parallel to bedding. In a few cases, detrital quartz grains are located inside semicontinuous fragmented rims.
Irregular particles . Spherule-rich samples from the Pering Mine region also contain irregular particles with flow-banded and vesicular textures similar to those found in the surface samples. No consistent textural differences were noted between the irregular particles in the Monteville Farm and Pering Mine regions.
Mica ribbons . In the Pering Mine region, a large number of grains contain mica ribbons. The ribbons have the same textures as described previously, again occurring in three distinct varieties. The textural positions of these varieties are (1) lining former vesicles, (2) at the boundary between the rim and the central clear spot, and (3) branching in a septarian geometry within the clear core. Overall, the ribbons are more abundant in samples from the Pering Mine region compared to samples from the Monteville Farm region. Within some elongate melt particles that show brittle fracturing and flattening, a preferred orientation of sericite crystals is sometimes developed that is length-slow parallel to bedding.
Pering Mine Region, Intraclast-Rich Subunit
There is only one known occurrence of the intraclast-rich subunit in the Pering Mine region (Table 1), where it is directly beneath a spherule-rich subunit that is 2.5 cm thick (Fig. 3). The spherule-rich subunit shows textures similar to those described above; the following descriptions apply only to this single known core occurrence of the intraclast-rich subunit.
As with the samples from the Monteville Farm region, the intraclast-rich subunit found in the Pering Mine region contains widely scattered pockets rich in spherules (Fig. 2) that are believed to be interstitial to large intraclasts consisting predominantly of dolomite. The former melt particles in these pockets show textures similar to those in the overlying spherule-rich units, but some textures are also present that were not observed in any other samples. Foremost among these is growth-banded quartz filling both vesicles and central clear spots within spherules. There is also a high abundance of carbonate cement comparable to the cement of intraclast-rich samples from the Monteville Farm region.
Sheet silicate replacement like that seen throughout the spherule-rich subunit in the Pering Mine region is also present but generally less common in the intraclast-rich subunit. A moderate amount of microcrystalline quartz is present and restricted to intergranular material. Some spherules show evidence of brittle deformation in the form of fractures in radial-fibrous spherule rims and shortening in the vertical direction. We attribute this to compaction, although an average of three contacts per grain indicates that this sand is much less compacted than the spherule-rich subunit. In addition to brittle shortening, long grain contacts and very thin intergranular selvages composed of sheet silicate phases provide evidence of compaction. Many spherules with blocky carbonate crystals at their centers exhibit carbonate-filled fractures in their rims that display optical continuity with crystals on one or both sides of the rim. Although the former melt particles in the intraclast-rich subunit from Pering Mine exhibit textures found both in the spherule-rich layer and in the outcropping intraclast-rich layer, the population of melt particles is more similar to the latter. No mica ribbons were observed in samples from the single core occurrence of the intraclast-rich subunit.
Based on both optical characteristics and some microprobe analyses of sample SF1–1 (Table 4), the spherules in the Monteville formation are now composed mainly of K-feldspar and sericite except where they are replaced by carbonate. Various lines of evidence indicate that these are all authigenic phases formed largely by replacement of both glass and crystalline phases formed from a melt. For example, the splash-form shapes of the spherules indicate they were originally molten and consisted largely of glass at the time of deposition, yet they now consist entirely of crystals. Another line of evidence is the fact that the carbonate-free spherule-rich samples consist almost entirely of K-feldspar and sericite. None of the rock types likely to make up the country rock in the target area (e.g., granite or basalt) consist of only these two minerals, indicating it is very unlikely that the spherules are isochemical with the original impact melt.
Moreover, microprobe analysis of spherules from sample SF1–1 indicates the K-feldspar has almost no Na2O (Table 4). Such a composition is typical of authigenic K-feldspar formed under low-temperature diagenetic conditions, e.g., via replacement (Simonson, 1992), but not via eutectic melt crystallization. This is not unusual as few if any original phases have survived to the present in Precambrian spherule layers, with the exception of skeletal spinels in a few layers (e.g., Byerly and Lowe, 1994). Even though they have been replaced wholesale and now consist largely of K-rich phases, it is likely that the spherules in the Monteville Formation were originally basaltic in composition. This is strongly implied by the fact that the internal textures in the lath-rich spherules bear a strong resemblance to partially crystallized basaltic melts formed both experimentally and naturally (Lofgren, 1977). Hassler and Simonson (2001) also noted basaltic tuffs in the Griqualand West Basin that showed similar replacement by K-feldspar.
Variation Between Intraclast-Rich and Spherule-Rich Subunits
The most noticeable contrast between the intraclast-rich and spherule-rich subunits is in the degree of compaction. Elongate, severely compacted grains showing evidence of brittle deformation (Category 1, Fig. 5) are much more abundant in the spherule-rich bed, whereas moderately compacted to spherical grains (Categories 2, 4, and 6) are more abundant within the intraclast-rich bed, although some also show brittle fracturing within their rims (Category 2). The fact that the number of contacts per grain is three times greater in the spherule-rich subunit than it is in the intraclast-rich bed also indicates the spherule-rich bed has undergone more compaction, despite the fact that the intraclast-rich subunit was buried to the same depth and experienced similar amounts of vertical compressive stress as the spherule-rich subunit. Higher average aspect ratios also indicate the spherule-rich subunit is more highly compacted than the intraclast-rich one (Fig. 12). Aspect ratios in samples from the intraclast-rich subunit fall between 1.4 and 2.4, whereas those from the spherule-rich subunit fall between 3.2 and 5.9. This also suggests around three times more vertical shortening in the spherule-rich layer.
The presence of localized shelter porosity in the intraclast-rich subunit indicates that the larger intraclasts formed a framework that could have protected the sand grains from some of the compressive stress. This, in turn, could have helped preserve intergranular porosity long enough to allow the emplacement of carbonate cement early enough in diagenesis for the sands in the intraclast-rich subunit to experience much less compactional deformation than those in the spherule-rich subunit (Fig. 4). The presence of abundant carbonate clasts within the intraclast-rich subunit could have aided in the early cementation of this subunit, as the clasts themselves may have provided a ready cement source. The absence of fine-grained matrix in this subunit is evidence for its deposition by a high-energy traction mechanism, such as a wave or unidirectional current, rather than via a debris-flow–type mechanism. The highest matrix percentage counted in the spherule-rich subunit was 23% by volume. There is a very small percentage of possible cement in the spherule-rich subunit and what there is consists mainly of sheet silicate. Even if the fine-grained matrix is detrital in origin, which we think is unlikely, it still would do little to help prevent pore volume loss during compaction, as sheet silicates are generally much easier to compact physically than sparry carbonate cement.
In contrast to the differences in compaction, the thicknesses of spherule rims are very similar in samples from the spherule-rich and the intraclast-rich subunits. Rim thicknesses in all samples range from 40 to 85 μm, with most falling into the range of 50 to 70 μm (Table 3). Likewise, the number of fractures per rim is rather consistent (Table 3) and does not vary systematically as a function of aspect ratio, contacts per grain, or any other measured quantity. This lack of correlation suggests the rim fractures formed via a mechanism unrelated to burial stress, presumably prior to compaction.
Variations Within Intraclast-Rich Samples
Within the Monteville Farm region, the relative abundances of different types of former melt particles are relatively consistent (Table 2), and the amount of compaction is generally low. There is a moderate decrease in the number of rim fractures per spherule from northeast (Monteville Farm) to southwest (Reid's Drift and Goudkop), but their aspect ratio stays relatively constant between 1.4 and 2.4 (Table 3). The subtle differences in former melt particle abundance may simply reflect varying degrees of cementation and replacement, but the diagenetic history is such that it is difficult to work out a detailed history of cementation. One problem is the abundance of pyrite, which not only obscures the size of carbonate crystals but obscures textures even more where pyrite is oxidized. The abundance of former melt particles within the material interstitial to the large intraclasts does vary within the region from a minimum of 20% to a maximum of 59% by volume. It is worth noting that the relative abundances of the different particle types were not affected to any great degree by this high level of disparity, which strongly suggests that former melt particles were not differentially replaced by carbonate to any significant degree during diagenesis. The fact that the intraclast-rich subunit samples from both core (SF1) and surface exposures (the Monteville Farm region) show similar textural character, including those features related to compaction (Fig. 13), further suggests that diagnostic ratios of different types of particles were similar over wide geographic areas and have not been significantly altered by surface weathering processes.
Within the Pering Mine intraclast-rich sample (SF1-1), a smaller amount of brittle grain deformation is seen than in the other samples, as it contains 7% fewer severely compacted elongate grains (Category 1) and 16% more spherical particles with botryoidal rims (Category 6) than any of the other samples. In general, if severely compacted elongate grains were originally spherical or ovoid, then the higher number of spherical grains in sample SF1-1 indicates a lesser degree of compaction. We interpret individual former melt particles in the SF1-1 core sample as having experienced a lesser degree of brittle deformation, on average, than those of the intraclast-rich subunit exposed at the surface. Nevertheless, the higher average of contacts per grain in the one core sample of the intraclast-rich subunit as compared to samples from surface occurrences indicates overall compaction was more extreme in the Pering Mine region.
The population of former melt particles in the intraclast-rich subunit in the SF1 core differs slightly from that of the Monteville Farm region in that irregular shapes increase by 13% in abundance in the Monteville Farm region. This could reflect a decrease in depositional energy from south to north inasmuch as irregular particles tend to be larger and more crystalline than spherules. This, in turn, would be consistent with evidence presented by Simonson et al. (1999) that the Monte-ville layer was deposited by high-energy waves and currents, possibly impact induced, directed roughly normal to the edge of the Kaapvaal Craton, which locally is roughly east-west. It is worth noting that in core SF1, the percentage of irregular particles in the intraclast-rich subunit is equal to that of the spherule-rich subunit (Table 2).
Variations Within Spherule-Rich Samples
Samples from the spherule-rich subunit all come from drill cores from the Pering Mine region, and they display a relatively uniform population of melt particles (Table 2), despite the degree of compaction generally increasing to the north (Table 3). The latter cannot be attributed to a progressive increase in load-induced stress, because the sediment package thins to the north (see Fig. 14 in Beukes, 1987). The average aspect ratio generally increases to the north, whereas rim fractures (Fig. 11) decrease slightly in abundance in that direction. Both these phenomena are thought to result from an increase in the thermal regime in the northern part of the study region where more dolerite dikes were observed in the cores than in those from the southern region. Generally, grains obscured by replacement are seen to decrease in abundance as the number of severely compacted grains increases. However, the spherule layer in the northernmost core (RF1) was only 4 m below a large dike or sill and displays both anomalously high numbers of elongate grains and one of the highest values for obscured textures (Table 2). The reason for this anomalous relationship could be that the highly elongate particles flattened by brittle deformation of their rims were replaced most readily due to the additional surface area resulting from fracturing. In contrast to the above trend, centrally located samples (e.g., those from P11, P9, and K1 cores) all behave in a similar fashion, showing moderate amounts of compaction and replacement. Transitionally located cores (V1 and TK1) show degrees of compaction intermediate between that of RF1 and that found in the centrally located cores. In summary, varying degrees of replacement and compaction were observed even within the small Pering Mine region, but they show subtle trends that may be the result of lateral thermal gradients.
Particles displaying mica ribbons (Fig. 7) are more abundant in samples from the spherule-rich subunit than in those from the intraclast-rich subunit (Fig. 14). Within a given sample, mica ribbons are less abundant in more highly compacted grains. Most mica ribbons occur in spherules possessing radial fibrous K-feldspar rims, the only exception being where they line vesicles. We believe vesicle linings formed as void-filling cements, and they occur in all types of grains. In spherules with K-feldspar rims, the ribbons not lining vesicles occur along contacts between rims and irregular clear spots and/or as radially branching networks inside irregular spots. These irregular clear spots are thought to represent glass cores that were devitrified or replaced by sericite and/or acicular K-feldspar during diagenesis (Simonson, 2003). We interpret the origin of the nonvesicular mica ribbons as follows.
It is likely that impact glass, including the glass cores of spherules, would still be hot enough from shock melting and/or atmospheric reheating to crack via the quick drop in temperature and resulting contractional stress once they came into contact with seawater. Given their crystallinity and radial crystal orientation, rims would be unlikely to contract as much, resulting in: (1) cores being pulled away from rims, and (2) cracks forming inside cores with a geometry similar to cracks seen in septarian nodules (Pettijohn, 1975). It has been proposed that thermal contraction cracking is a mechanism for microtektite fragmentation in Cenozoic strewn fields (Glass et al., 1997). As microtektites consist entirely of glass and possess no rim, complete disarticulation of a microtektite is more likely than in the case of a microkrystite. If spherules possessing internal microfractures are reworked by a high-energy event as those in the Monteville layer clearly were, the possibility of fragmentation would increase. Perhaps the presence of unfractured crystalline rims helped many spherule cores to avoid fragmentation during deposition. Preexisting internal fractures should also make it easier for spherule cores to dis-aggregate under burial-induced pure shear stress. Microcracks could also serve as conduits for fluid entry and help guide parallel fibrous replacement via nucleation along cracks in a manner analogous to axiolitic replacement of volcanic glass shards or the formation of mesh texture in serpentinites. A similar effect could take place along the edge of the glass core where it is in contact with the crystalline rim. This replacement is likely to have resulted in the parallel fibrous textures we describe as isopachous mica ribbons.
Deformation Textures and Modes of Vertical Shortening
About half of the splash-form population at each site experienced brittle compaction, as evidenced by rim fractures showing varying degrees of displacement, often resulting in disaggregation into fragment clusters. The small groups of fragments are oriented in a way that, along with their rim structures, suggests fragmentation of spheroidal to elongate splash forms with relatively thin rims. In the more highly compacted spherule-rich subunit, we have documented a direct correlation between thickness of spherule rim and degree of compaction; specifically, individual spherules with thicker rims are likely to have lower aspect ratios in all samples (Table 3). Spherule rims show brittle fractures that indicate they are acting as rigid shells enclosing a much weaker central core of sheet silicate or possibly a void in some cases.
Another dominant mechanism resulting in long and concavo-convex grain boundaries as well as bent but not fractured rim segments is diffusional mass transfer (DMT) of minerals such as K-feldspar and quartz (Passchier and Trouw, 1996). The difference in behavior between central clear spots now composed of sericite versus fracturing of rims made of feldspar presumably reflects different responses to stress diagnostic of different minerals. Sheet silicate crystals oriented length-slow parallel to bedding suggest that reorientation or oriented growth are shortening mechanisms that occurred within the cores of spherules either through DMT or simple mechanical compaction. Sheet silicates deform by slip limited to discrete surfaces. Therefore, they show abundant evidence of accommodation mechanisms other than DMT (Passchier and Trouw, 1996). So, reorientation and slip-dominated mechanical compaction are more likely to have occurred in the sheet silicate cores.
Rim fractures show different geometries on different grains, and two geometries are dominant, with spherules containing either one or both types. The first geometry resembles the “eggshell” fracturing of a hollow spheroidal particle (Wilkinson and Landing, 1978) and is characterized by wedge-shaped fractures that can taper either into or out of the center of a spherule (Fig. 6). “Eggshell” fractures only form where a central void is present into which the fracturing rim can be displaced. Detrital quartz grains located inside rims showing the “eggshell” geometry confirm the fact that voids were present in such spherules; these quartz grains are usually accompanied by the chloritic sheet silicate. The second geometry consists of slip-dominated planar fractures at high angles to the rim. In such particles, offset is almost entirely parallel to the fracture, whereas extension perpendicular to the fracture is insignificant (Fig. 11). In this case, internal volume loss via DMT or slip-dominated reorientation of sheet silicate crystals could accommodate shortening without the need for voids, thereby avoiding an “eggshell”-type geometry. These rim microfaults are often accompanied by ovoid zones of recrystallized sheet silicates within the rim in the areas experiencing the most shortening (Fig. 10), indicating DMT aided migration of sheet silicate material, most likely from within the core toward a location in the rim. These two different shortening mechanisms could have worked in proportionally variable combinations.
SUMMARY AND CONCLUSIONS
Spherules in the Monteville Formation are now composed primarily of authigenic K-feldspar, muscovite (sericite) and chloritic phases, and carbonate. All of these phases are thought to be the result of replacement and therefore do not reflect the original mineralogy of the melt particles. However, pseudomorphed crystal textures within the spherules suggest that they were originally basaltic in composition. The fact that regionally associated tuffs thought to be basaltic in composition are replaced in a very similar fashion supports this interpretation. Since the bulk of the mass in impact ejecta is derived from the target rock, this in turn suggests formation of the Monteville spherules by an impact into oceanic crust.
Various lines of evidence indicate the intraclast-rich subunit experienced less compaction than the spherule-rich subunit, e.g., its higher content of interstitial carbonate spar is interpreted as cement or a replacement of cement. This lower degree of compaction within the intraclast-rich subunit leads us to believe it is the least altered and most closely representative of the original population of former melt particles. However, the fact that the number of rim fractures shows no increase as the degree of compaction increases between the two subunits suggests that the particle rims experienced structural weakening in equal measure throughout the study area prior to deposition. The formation of cracks via thermal shrinkage in glass cores and differential shrinkage between radial fibrous rims and glass cores could provide the mechanism for this structural weakening and resulting fracturing early in diagenesis. The microcracks in some spherules then served as conduits for fluid entry and localized replacement by parallel fibrous sheet silicates to form mica ribbons. In other spherules, the microcracks were apparently conduits for dissolution instead, helping to create voids that enabled “eggshell”-type compaction. In still other spherules, diffusional mass transfer (DMT) and/or crystal reorientation enabled grains to shorten via microfaulting without obvious fragmentation of the rim. Fracturing of rims and reorientation or DMT of centrally located sheet silicate minerals indicates some compaction either postdates or was contemporaneous with the later stages of replacement. Despite all these modifications, the former melt particle populations within both the intraclast-rich and the spherule-rich subunits show only slight disparities over distances on the order of 100 km, both within and between subunits.
Implications for Intercontinental Correlation
Simonson et al. (1999) discussed the possible correlation of the Monteville layer with spherule layers in the Jeerinah, Wittenoom, and/or Carawine formations in the Hamersley Basin based on age constraints available at the time, as well as sedimentologic and gross petrographic similarities. Since this paper was published, another outcrop of the Jeerinah layer was found at the Hesta locality in the Hamersley Basin (Simonson et al., 2001; Hassler et al., 2005). Additional occurrences in drill cores are described by Rasmussen and Koeberl (2004). This layer shows some striking similarities to the Carawine layer, particularly an abundance of irregular melt particles. To further test the proposed correlations, a sample from the Hesta occurrence was point counted using the methods described above, and it shows striking similarities to the Monteville spherule layer (Table 2). Not only are the relict textures that the spherules inherited from their molten state very similar, they seem to have experienced similar diagenetic histories as well, most notably the formation of isopachous mica ribbons in spherules. Our studies of the Monteville layer suggest that the melt particles in spherule layers can be relatively homogeneous on a scale of a few hundred kilometers and retain a significant amount of textural information despite widespread diagenetic alteration. The similarities with the Jeerinah layer in Australia indicate this may also be true over much larger distances and include similarities in diagenetic as well as primary features. It may provide additional evidence that the Pilbara and Kaapvaal cratons were both part of a single depositional basin at the time the Monteville layer formed, as some workers have suggested (e.g., Cheney, 1996).
Field work was supported by grants from the National Geographic Society and Oberlin College, Oberlin, Ohio, United States. We thank Pering Mine (Pty) Ltd. and Gold Fields of South Africa Ltd. for permission to examine cores through the Monteville Formation. Former Oberlin students Paul Jambor and Brooke Wilkerson provided field assistance, and Michael Cardiff and Trista Thornberry helped in the laboratory. We also thank Jan Smit and the Smithsonian Institution for the loan of samples from the K/T boundary layer. We are especially indebted to Dr. Nic Beukes of Rand Afrikaans University for introducing us to the field area and making the arrangements that made this study possible. Dr. Jens Gutzmer of the University of Johannesburg performed the electron microprobe analyses given in Table 4. The manuscript benefited greatly from careful reviews by Drs. Burkhard Dressler and Christian Koeberl.
Figures & Tables
Processes on the Early Earth
- electron probe data
- Hamersley Basin
- K-T boundary
- lower Paleocene
- mass transfer
- mineral composition
- point counts
- South Africa
- Southern Africa
- stratigraphic boundary
- Upper Cretaceous
- Western Australia
- Griqualand West Basin
- Pering Mine
- Monteville Formation
- Monteville Farm