Charles W. Byers

Tidal deposition in the basal Upper Cambrian Mt. Simon Formation in Wisconsin

The Upper Cambrian Mt. Simon Formation (0-65 m thick) is a basal quartz arenite exposed in westcentral Wisconsin. A detailed field investigation of the physical and biogenic sedimentary structures of the Mt. Simon has led to the recognition of three distinct lithofacies. The lower one unconformably overlies Precambrian basement rocks. It consists of medium- to very large-scale sets of tabular and trough cross-bedded, medium- to very coarse-grained sandstone and pebbly sandstone with minor intercalated horizontal beds of very fine- to medium-grained sandstone, siltstone, and shale. Sparse examples of Skolithos and Arenicolites are present. This facies consists of a very thin sequence of possible braided-fluvial and marine foreshore deposits, overlain by probable marine shoreface and tidal channel deposits. Much of the facies seems to represent shallow subtidal deposition in a relatively high-energy regime. The middle lithofacies consists of two distinctly different subfacies, which probably were deposited in a low tidal flat setting. The higher-energy subfacies consists of small- to medium-scale sets of tabular and trough crossbedded, fine- to coarse-grained sandstones containing distinct zones dominated by Skolithos and Arenicolites . This subfacies probably represents deposition in meandering tidal channels. The lower-energy subfacies consists of thinbedded, horizontally-laminated and ripple cross-laminated, very fine- to medium-grained sandstone, siltstone, and shale, with common specimens of Cruziana, Rusophycus, and Planolites . This subfacies probably represents deposition on lower-energy tidal flats adjacent to the tidal channels. The upper lithofacies consists predominantly of structureless, densely bioturbated, very fine- to coarse-grained sandstone containing abundant specimens of Skolithos . The upper few meters of the facies consists of small- to medium-scale sets of trough cross-bedded, very fine- to coarse-grained sandstone with layers of disarticulated valves of the brachiopod Obolus . The upper facies probably represents deposition on tidal flats, perhaps in a midtidal flat setting, characterized by slower sedimentation rates, a correspondingly higher degree of bioturbation, persistent reworking of shelled macrobenthos, and periodic subaerial exposure. The Mt. Simon Formation is interpreted as a largely progradational (regressive), shoaling- and fining-upward tidal sequence. A marine interpretation is supported by the widespread occurrence of marine trace fossils within this unit. Evidence for a tidal origin is seen in the presence of unimodal cross-strata associated with reactivation surfaces, compound cross-strata, numerous scour and truncation surfaces lined with intraformational conglomerates, common clay drape laminae separating sets of cross-strata, interference and flat-topped ripple marks, and desiccation cracks. Sedimentation continued without apparent interruption as the overlying Eau Claire Formation was deposited. also under tidal influence. Recent reinterpretations of other basal Cambrian cratonic quartz arenites, together with this new interpretation for the Mt. Simon Formation, suggest that the long-held concept of basal transgressive sandstones deposited as blankets across the craton may be too simplistic, for deposition in braided-fluvial, marginal marine (tidal flat-tidal channel), and marine foreshore and shoreface environments seems indicated.

Abundant and diverse early Paleozoic infauna indicated by the stratigraphic record

Marine benthic animals have lived within the sediment since the late Precambrian. An abundant early Paleozoic infauna is indicated by bioturbated zones and diversity of deposit-feeder and suspension-feeder trace-fossil genera. Burrow depths extended tens of centimetres below the sediment-water interface. Trace-fossil evidence of an abundant early Paleozoic infauna conflicts with evidence from the shelled fauna, which lacks infauna. This requires reinterpretation of early Paleozoic community structure and raises new questions about interactions between early Paleozoic soft-bodied and skeletonized animals.

Geological Significance of Marine Biogenic Sedimentary Structures

The “geological significance” of traces encompasses a huge range of topics; to survey the field even briefly would require an entire book and probably multiple authors. Fortunately a major review volume was published within the last decade (Frey, 1975), and a diversity of topics were addressed. In addition, the revised Volume W of the Treatise on Invertebrate Paleontology (Hantzschel, 1975), which covers the systematics of trace fossils, also appeared recently. These two books have served to provide a solid base for research in trace fossils in terms of current systematics and concepts of classification, how traces are made and preserved, and the relation of traces to sedimentology, biostratigraphy, paleontology, and paleoecology.

Paleoenvironments of Mowry Shale (Lower Cretaceous), Western and Central Wyoming: GEOLOGIC NOTES

The Lower Cretaceous Mowry Shale of western and central Wyoming contains three distinct facies, each of which is characterized by a unique suite of physical and biogenic sedimentary structures: (1) laminated mudstone, (2) bioturbated mudstone, and (3) bioturbated sandstone. We interpret the facies to represent the following marine biotopes (Schafers terminology): (1) lethal isostrate--quiet, anaerobic, >150 m deep; (2) vital isostrate--quiet, marginally aerobic, 15 to 150 m deep; and (3) vital heterostrate--agitated, fully aerobic, <15 m deep. Bentonite datum planes show that the boundaries between facies are time transgressive, and indicate a west-to-east progradation of the shoreline and lateral filling of the Mowry sea. An east-dipping paleoslope inclined at 0 76;13^prime has been estimated. The depositional topography of the Mowry basin as reconstructed from the facies pattern shows that the thickest accumulation of sediment occurred along the western margin where water was shallowest, whereas in the deeper water basin center a much thinner section was deposited. Thus, isopach patterns of the Mowry in no way reflect the original depositional topography. The significance of this study for petroleum geology is threefold: (1) a facies sequence is used to reconstruct the depositional basin configuration of a known source rock; (2) the facies correlate with previously published regional variations in organic carbon concentration in the Mowry; and (3) the facies also correspond approximately to facies based on electrical resistivity.

Sequence Stratigraphy of the Lower Ordovician Prairie Du Chien Group on the Wisconsin Arch and in the Michigan Basin

Mixed carbonate-siliciclastic sediments of the Prairie du Chien Group were largely deposited in shallow tropical seas. Sedimentologic indices of shallow-water deposition and a moderately diverse Early Ordovician macrofauna and mid-continent conodont fauna indicate that shallow-marine conditions prevailed across the Wisconsin arch and Michigan basin throughout most of Prairie du Chien deposition. Although the Wisconsin arch and Michigan basin were weakly active structural features, tectonism does not appear to have appreciably influenced water depths. The Michigan basin was not a bathymetric basin during the Early Ordovician as it became during the Silurian. The Prairie du Chien Group contains two major depositional sequences, the Oneota and Shakopee formations, both of which are bounded by type 1 sequence boundaries. On the Wisconsin arch, type 1 sequence boundaries are associated with karsting and silicification of underlying carbonates, indicating unconformity development during prolonged subaerial exposure. In the central Michigan basin, formation contacts are sharp and appear disconformable. The contacts between the two lithostratigraphic members comprising each formation in outcrop do not appear to be subaerial unconformities and are interpreted as type 2 sequence boundaries.

Anatomy of an embayment in an Ordovician epeiric sea, Upper Mississippi Valley, USA

The integration of stratigraphic, geochemical, and biostratigraphic data from Middle Ordovician carbonates and shales indicates that the North American epeiric sea was partitioned into shelf areas with distinct characteristics. The Upper Mississippi Valley part of the epeiric sea was appraised by using regionally traceable and geochemically “fingerprinted” K-bentonites, as well as detailed lithologic correlation. In the Midcontinent, the Decorah Formation records a time of high clastic sediment influx and abundant freshwater runoff from the Transcontinental Arch that created a salinity-stratified water column and led to episodic dysoxia. Later, relative flooding of the clastic source areas greatly reduced both the clastic sediment and freshwater runoff. As a result, the salinity stratification broke down, oxygenating the seafloor and permitting carbonates to form. Associated with this change, clarity of the water improved and the photic zone expanded, allowing seasonal blooms of Gloeocapsomorpha prisca to occur, resulting in increased burial of organic matter. The increase in G. prisca and total organic carbon coincided with, but lagged behind, a regional δ 13 C excursion. In addition, the timing of the initiation of the isotopic anomaly is different across the studied area, suggesting that local environmental conditions influenced the isotopic record. Data presented in this study support the partitioning of distinct areas within epeiric seas and the importance of this setting in storing inorganic and organic carbon and recording environmental and biological changes.

Depositional Mechanisms and Organic Matter in Mowry Shale (Cretaceous), Wyoming

Four lithofacies, which accumulated under different depositional conditions, can be distinguished in the Lower Cretaceous Mowry Shale of Wyoming. Type and amount of organic matter in these lithofacies are governed largely by sediment transport mechanisms. Nearshore silt and mud containing terrestrial organic matter were deposited in a prodelta environment by tractive processes and from suspension. These sediments, along with terrestrial organic matter, were redistributed by waning bottom flows over a dysaerobic slope; pelagically deposited muds containing marine-derived organic matter accumulated between flow events. Muds farthest offshore accumulated in dysaerobic and anaerobic water by pelagic settling in a region unaffected by bottom currents; these muds contain predom nantly marine-derived organic matter. Petroleum source potential increases in a southeastward direction across west and central Wyoming. This trend results from differential input of marine and terrestrial organic matter, clastic dilution, and postdepositional biodegradation.

Trace Fossils and Sedimentologic Interpretation--McGregor Member of Platteville Formation (Ordovician) of Wisconsin

ABSTRACT The McGregor Member is a dominantly micritic carbonate unit, approximately 10 m thick in southwestern Wisconsin, which displays considerable uniformity laterally in terms of sedimentary structures, fossils, and trace fossils. Two trace fossil genera are prominent and help to interpret the units depositional history. The base of the Member is a sharp contact exhibiting Trypanites and is here interpreted as a submarine hardground. Within the Member Chondrites is present in profusion. Burrows are outlined by dark pyritic rims on fresh rock surfaces; weathering oxidizes the pyrite to limonite, causing the burrows to lose definition and eventually to disappear into a diffuse limonitic stain. On weathered outcrops the pervasive Chondrites bioturbation is cryptic. In areas where the McGregor is partially dolomitic, the dolomitization mostly affected burrows and the sediment immediately surrounding them, suggesting control of the diagenetic fluids by the permeability variations of the bioturbate zones. Chondrites show varying amounts of compression relative to the nodular bedding planes of the McGregor: burrows are circular to elliptical within micritic lenses but flattened within the shaly partings between lenses. The compression demonstrates that the bedding is secondary, probably produced by differential compaction of a thoroughly bioturbate, quiet-water mud; this interpretation is at odds with previous workers who ascribed the wavy bedding to ripples and inferred that the depositional environment was shallow and agitated. The presence of a basal hardground across the entire study area (110 km) and the dominance of Chondrites everywhere within the unit suggests that no significant environmental gradient, such as water depth or intensity of physical reworking, was pr sent, even across the Wisconsin Arch.

High-Resolution Sequence Stratigraphic Analysis of the St. Peter Sandstone and Glenwood Formation (Middle Ordovician), Michigan Basin, U.S.A.

The Middle Ordovician St. Peter Sandstone and Glenwood Formation (Ancell Group) represent a significant target for gas exploration at the base of the Tippecanoe sequence in the Michigan basin. Core and well log data show that the St. Peter-Glenwood interval contains numerous carbonate units that provide the basis for both regional correlation and subdivision of the section into at least 20 high-frequency sequences. The temporal resolution afforded by these sequences allows a detailed analysis of sediment partitioning as the basin evolved. The spatial distribution of the basal sequences illustrates the pronounced east-to-west onlap of the Wisconsin arch. An abrupt increase in sequence thickness upsection indicates that a major episode of basin-centered subsidence began during middle St. Peter deposition and continued through the deposition of the Glenwood Formation. The upper sequences show a significant beveling of the Glenwood Formation and the top of the St. Peter Sandstone in the north, south, and southeast areas of the basin prior to deposition of the overlying Black River carbonates. Although eustatic sea level changes were undoubtedly operating at several scales, the facies distribution of this mixed clastic/carbonate system also documents significant changes of local and regional tectonics.

Shelf Sandstones in The Mowry Shale: Evidence for Deposition During Cretaceous Sea Level Falls

Two previously undescribed sandstone beds in the Lower Cretaceous Mowry Shale in north-central Wyoming form elongate, retrogradational bodies, meters thick and tens of kilometers in length, that are encased in shale. The sandstones were deposited under different depth and oxygen conditions than were the shales. The sands are extensively bioturbated and contain wave-induced features (oscillation ripples and hummocky stratification), whereas the surrounding muds contain few biogenic structures and lack evidence of wave motion. Mowry sandstone units are interpreted to have been deposited by storms in shallow (10 to 30 m), oxygenated water. The shales that encompass these sandstones were deposited largely from turbid bottom flows in dysaerobic and anaerobic water. A drop in sea level is postulated to have promoted deposition of each sandstone bed on sections of the shelf where muds had accumulated previously below wave base in oxygen-poor water. During low water level this region was exposed to well-oxygenated water and to the effects of storms. Subsequent sea level rise shifted the locus of sand deposition, wave base, and the intersection of oxygen boundaries with the seafloor in a shoreward direction. The retrogradational geometry of each sand body is explained by the shoreward migration of sand deposition during sea level rise.