Jürgen Schieber

POSSIBLE INDICATORS OF MICROBIAL MAT DEPOSITS IN SHALES AND SANDSTONES : EXAMPLES FROM THE MID-PROTEROZOIC BELT SUPERGROUP, MONTANA, U.S.A.

Abstract It has been suspected for some time that microbial mats probably colonized sediment surfaces in many terrigenous clastic sedimentary environments during the Proterozoic. However, domination of mat morphology by depositional processes, post-depositional compaction, and poor potential for cellular preservation of mat-building organisms make their positive identification a formidable challenge. Within terrigenous clastics of the Mid-Proterozoic Belt Supergroup, a variety of sedimentary structures and textural features have been observed that can be interpreted as the result of microbial colonization of sediment surfaces. Among these are: (a) domal buildups resembling stromatolites in carbonates; (b) cohesive behaviour of laminae during soft-sediment deformation, erosion, and transport; (c) wavy–crinkly character of laminae; (d) bed surfaces with pustular–wrinkled appearance; (e) rippled patches on otherwise smooth surfaces; (f) laminae with mica enrichment and/or randomly oriented micas; (g) irregular, curved–wrinkled impressions on bedding planes; (h) uparched laminae near mud-cracks resembling growth ridges of polygonal stromatolites; and (i) lamina-specific distribution of certain early diagenetic minerals (dolomite, ferroan carbonates, pyrite). Although in none of the described examples can it irrefutably be proven that they are microbial mat deposits, the observed features are consistent with such an interpretation and should be considered indicators of possible microbial mat presence in other Proterozoic sequences.

Diagenetic origin of quartz silt in mudstones and implications for silica cycling

Mudstone—the most abundant sedimentary rock type, composed primarily of clay- or silt-sized particles—contains most of the quartz found in sedimentary rocks. These quartz grains, which are chemically and mechanically resistant and therefore preserve their characteristics well, have long been considered to be derived from the continental crust. Here we analyse quartz silt from black shales in the eastern USA, dating back to the Late Devonian period (about 370 million years ago), using backscattered electron and cathodoluminescence imaging and measure oxygen isotopes with an ion probe. Our results indicate that up to 100% of the quartz silt in our samples does not originate from the continental crust. Instead, it appears to have precipitated early in diagenesis in algal cysts and other pore spaces, with silica derived from the dissolution of opaline skeletons of planktonic organisms, such as radiolaria and diatoms. Transformation of early diatoms into in situ quartz silt might explain the time gap between the earliest fossil occurrences of diatoms about 120 Myr ago and molecular evidence for a much earlier appearance between 266 or even 500 Myr ago. Moreover, if many other mudstone successions show similarly high proportions of in situ precipitated—rather than detrital—quartz silt, the sedimentary record in mudstones may have been misinterpreted in the past, with consequences for our estimates of palaeoproductivity as well as our perceptions of the dynamics and magnitude of global biogeochemical cycling of silica.

Evidence for high-energy events and shallow-water deposition in the Chattanooga Shale, Devonian, central Tennessee, USA

Abstract The Upper Devonian Chattanooga Shale of central Tennessee, a classical black shale, was deposited in an epicontinental setting, west of the Appalachian foredeep. Its finely laminated and highly carbonaceous nature is commonly interpreted to indicate deposition in comparatively deep and stagnant water. Interbeds of bioturbated greenish-gray shale, indicating oxygenated bottom waters, are commonly ascribed to pycnocline fluctuations. However, laminated fine sand and silt and hummocky cross-stratification (HCS) at the base of some of these beds indicates interaction of storm waves with the seabed, and suggests that greenish-gray shale beds are post-storm mud drapes. Other interesting features are inclined-undulose erosion surfaces that are conformably overlain by shale beds, sets of inclined shale beds that suggest low-angle cross-bedding, and clearly and uniformly developed alignment of clay particles (magnetic fabric studies). These observations show that the seabed was at times subject to prolonged erosion by bottom currents (erosion surfaces), agitation and reworking by storm waves (HCS and greenish-gray shale beds), and sediment transport by long-lived bottom currents (particle alignment). The epicontinental sea setting and the presence of HCS and other storm-produced features suggest a relatively shallow water depth (possibly only a few tens of meters). Together with abundant evidence of variably strong bottom currents and bioturbation of black and gray shale beds this suggests that abundant planktonic organic matter production rather than stagnant bottom waters are the primary cause for black shale formation.

A sulfur isotope study of pyrite genesis: The mid-proterozoic Newland formation, belt supergroup, Montana

Abstract Different generations of sedimentary pyrite from the Mid-Proterozoic Newland Formation, USA, have been analysed for their sulfur isotopic compositions. The results indicate bacterial sulfate reduction as the pyrite forming process. The δ 34 S values for early diagenetic pyrite, around −14%., are in contrast to dominantly more positive values for many other Middle Proterozoic units. A progressive reduction of sulfate availability during diagenesis can be recognized by an increase in 34 S content (Rayleigh Distillation) as well as through detailed petrographic observations. Contemporaneous seawater had a sulfur isotopic ratio between +14 and +18%. as measured from sedimentary barite within the unit.

Significance of styles of epicontinental shale sedimentation in the Belt basin, Mid-Proterozoic of Montana, U.S.A.

Abstract Within the strongly shale dominated sediment fill of the Mid-Proterozoic Belt basin a large variety of shale types can be distinguished. Sedimentological investigations of several formations have yielded valuable data about lateral associations of shale types as well as about the principal mechanisms of shale deposition. Depositional environments of shales in the Belt basin range from red bed mudstones of ancient flood plains to deep water mudstones in a turbidite setting. Graded silt/mud couplets are ubiquitous in most shales of the Belt basin. However, they differ in detail and can be related to a variety of depositional processes, such as low-density turbidity currents, sheet floods, storms, and wave reworking. In several formations there is evidence that microbial mats colonized the sediment surface and probably protected the sediment surface from erosion. Shales from comparable Phanerozoic settings generally lack the silt/mud couplets that are so commonly observed in shales from the Belt basin and other Proterozoic epicontinental basins. This difference probably reflects the proliferation of bioturbating organisms in the Phanerozoic. The absence of benthic microbial mats in Phanerozoic shales is probably due to the evolution of metazoan grazers towards the end of the Proterozoic.

A combined petrographical-geochemical provenance study of the Newland Formation, mid-Proterozoic of Montana

A provenance study was conducted on the Mid-Proterozoic Newland Formation, in which petrographical features of sandstones and geochemical characteristics of shales were integrated to arrive at an internally consistent interpretation. Sandstones of the Newland Formation are typically arkosic sands and arkoses with very-well-rounded quartz and feldspar grains and only minor amounts of extrabasinal rock fragments. The predominant feldspar types are K-spar and microcline, feldspar grains are smaller than quartz grains, and feldspars show little alteration due to weathering. Detrital modes of Newland sandstones (QFL diagrams) indicate that they were derived from a stable cratonic source. These petrographical features imply a source area dominated by granites and granitoid gneisses, semi-arid to arid climate, tectonic quiescence, and overall peneplain conditions. Shales of the Newland Formation are dominated by illite, quartz silt, and fine crystalline dolomite. They have small La/Th rations, relatively large Hf contents, and small contents of Cr, Co, and Ni, all indicative of derivation from crust of granitic composition. Small Tio 2 /Al 2 O 3 ratios also suggest source rocks of granitic composition. The average chemical index of alteration (CIA) for Newland shales is 71.8, which in light of the probable granitoid source indicates modest amounts of chemical weathering. Relatively large SiO 2 contents and large K 2 O/Na 2 O ratios reflect derivation from stable cratonic areas and tectonic quiescence. Thus, in general, the petrography of sandstones and geochemistry of shales provides the same provenance clues for the Newland Formation. One notable discrepancy between the two approaches is that the sandstones indicate an arid to semi-arid climate with very minor chemical weathering, whereas the CIA of the shales indicates at least modest amounts of chemical weathering. This indicates on one hand the need to better calibrate the CIA with a large variety of muds from modern climatic settings, and on the other hand the possibility that this discrepancy is due to transport segregation.

Pyrite mineralization in microbial mats from the mid-Proterozoic Newland Formation, Belt Supergroup, Montana, U.S.A.

Abstract The Mid-Proterozoic Newland Formation, a shale-dominated unit of the Belt Supergroup, was deposited in an eastern extension of the Belt basin, the Helena embayment. A variety of different shale facies types can be distinguished. Of particular interest for this study is a shale facies that has been interpreted to be the result of microbial mat growth (resulting in carbonaceous shale beds) interrupted by storm deposition (causing deposition of graded silt/mud couplets). Alternation of carbonaceous beds with silt/mud couplets gives these shales a characteristic striped appearance. Along the basin margins a pyrite-rich sub-facies of these striped shales is found locally, consisting of laminated pyrite beds that alternate with non-pyritic silt/mud couplets. Laminated pyrite beds in pyritic striped shales are interpreted as mineralized microbial mats because of wavy-crinkly internal laminae and because of the direct association with unmineralized striped shales that contain microbial mat deposits. Excess iron in pyritic shale horizons was probably supplied by terrestrial runoff in colloidal form. Iron hydroxides, introduced by rivers into basin marginal lagoons, flocculated, and were then incorporated into microbial mats and reduced to pyrite upon burial.

Determination of Basinwide Paleocurrent Patterns in a Shale Succession from Anisotropy of Magnetic Susceptibility (Ams): A Case Study of the Mid-Proterozoic Newland Formation, Montana

ABSTRACT In shales of the Mid-Proterozoic Newland Formation, magnetic fabrics can be used as indicators of paleoflow direction. The long axis of the anisotropy of magnetic susceptibility (AMS) ellipsoid parallels paleoflow and is inclined in the upcurrent direction (Schieber and Ellwood 1988). To apply the AMS paleocurrent method to shales on a basinwide scale, oriented samples from the two members of the Newland Formation and its lateral equivalents were collected over the entire southeastern Belt basin. For the lower member of the Newland Formation the data show a parallel-bipolar and north-south-oriented paleoflow pattern. The pattern for the upper member of the Newland Formation is more complex: it indicates the presence of a northern and southern shoreline from where sediment is transferr d deeper into the basin (parallel-unimodal pattern), as well as a central zone of overlap with a parallel bipolar pattern. The results of the study show that AMS-based paleocurrent patterns support paleogeographic reconstructions based on stratigraphic and sedimentologic studies, and that tectonic deformation did not significantly alter primary fabrics. In the upper member of the Newland Formation, shore-normal vs. shore-oblique sediment transport for sandy and muddy storm deposits, respectively, is in agreement with a recently proposed depositional model for storm deposits (Duke et al. 1991).

A role for organic petrology in integrated studies of mudrocks: examples from Devonian black shales of the eastern US

Abstract Over the years, the study of mudrocks has lagged far behind that of other lithologies, a circumstance that is in part due to their fine-grained nature, and in part due to economic realities. Yet, within petroleum systems mudstones are usually the main source of hydrocarbons and typically are also important as hydrocarbon seals. Recent work on Late Devonian mudstones from the eastern US shows that much progress can be made through an integration of outcrop study, macro- and micropaleontology, ichnofossils, gamma ray spectroscopy, microscopic examination of thin sections in transmitted and reflected light, electron microscopy (SEM, BSE), electron microprobe, carbon and sulfur isotopes, and organic geochemistry. Erosion surfaces within this black shale succession have been traced over large distances and provide the foundation for a sequence stratigraphic re-interpretation of these rocks. Evidence of storm wave reworking of the seabed, as well as the realization that benthic colonization was much more widespread than previously believed, suggest that anoxic conditions in the water column were not a controlling factor in the accumulation of the large quantities of organic matter found in these shales. These distal Devonian shales accumulated slowly and allowed accumulation of large proportions of organic matter. In modern settings of abundant organic matter accumulation, the original material is broken down by bacteria within a matter of months into a mass of largely unidentifiable organic particles and extracellular bacterial slime. Although one can still find identifiable material within this mass, slime and amorphous material strongly dominate. After burial and maturation this material turns into the various organic macerals that organic petrologists are accustomed to describe. One might wonder in this context, how much information about the origin of a given mudstone unit can we hope to extract through organic petrology? In order to illustrate how organic petrology of mudrocks can contribute to their better understanding, Late Devonian black shales of the eastern US will be examined from a sedimentological perspective. Combining sedimentologic and geochemical data with basic observations on organic petrology, illustrates how the latter can contribute to more realistic scenarios of black shale genesis.

Anomalous iron distribution in shales as a manifestation of “non-clastic iron” supply to sedimentary basins: relevance for pyritic shales, base-metal mineralization, and oolitic ironstone deposits

In previous investigations, nearshore pyritic shale horizons in the Mid-Proterozoic Newland Formation were interpreted to be due to “non-clastic” colloidal iron supply by streams. New data on the chemical composition of shales in the Newland Formation support this interpretation. In these shales, Fe and Al show a positively correlated trend that intercepts the Fe axis above the origin. These relationships suggest control of Fe by clays (via iron oxide coatings on clay minerals), and the presence of an additional, “non-clastic iron” component. Shales from stratigraphic intervals during which pyritic shale horizons were deposited plot above the Fe/Al trend typical for the remainder of the Newland Formation.Pyritic shale horizons in sediments are favourable hosts for base-metal deposits of the pyrite-replacement type. Fe/Al relationships as found in the Newland Formation may help to identify stratigraphic horizons in other sedimentary basins that contain pyritic shale horizons and potentially base-metal mineralization.