Robert H. Dott

Distinctions and uses of stratification types in the interpretation of eolian sand

ABSTRACT Eolian and subaqueous cross-strata cannot be distinguished reliably by many commonly cited criteria. They can, however, generally be distinguished by the characteristics of their component types of stratification, which represent processes of sorting and transport of the grain population on dunes. Eolian dune stratification types consist of grainflow cross-strata, grainfall laminae, and wind ripple-generated climbing translatent strata. Some of these types, especially translatent strata, have characteristics unique to the eolian realm. These same stratification types are found to compose some ancient cross-strata, and their occurrence confirms the eolian interpretations of pans of the Entrada (Jurassic), Navajo (Triassic-Jurassic), and Galesville (Cambrian) Formations, a well as revealing emergent islands in the Curtis Formation (Jurassic), previously considered to be totally marine in origin. Stratification types show a characteristic distribution on modern eolian dunes and differ in their relative abundances and structure on dunes of differing size and kind. These same relations allow some estimates of the type, shape, and original height of ancient dune deposits, as well as influencing the occurrences of surface features such as tracks and tipple forms. The geological record of stratification types and other dune features is greatly affected, however, by the extent of the post-depositional truncation of dunes.

Stranded on a Late Cambrian shoreline: Medusae from central Wisconsin

Fossilized impressions of soft-bodied organisms are exceptionally rare in coarse-grained strata. Fossilized mass-stranding events of soft-bodied organisms are even rarer. The Upper Cambrian Mt. Simon–Wonewoc Sandstone in central Wisconsin contains at least seven horizons characterized by hundreds of decimeter-sized impressions of medusae; these represent one of only two fossilized mass-stranding deposits. Medusae exhibit features nearly identical to those observed in modern scyphozoan strandings, including impressions of subumbrellar margins and gastrovascular cavities. This deposit provides insights about soft-tissue preservation in Phanerozoic marginal marine sediments, and suggests that large soft-bodied pelagic organisms were abundant in Cambrian seas.

Large-scale traction-produced structures in deep-water fan-channel conglomerates in southern Chile

The Lago Sofia conglomerate lenses in the Upper Cretaceous Cerro Toro Formation of southern Chile represent a channel-levee facies on a north-to-south-oriented deep-sea fan that formed between the rising cordillera to the west and the South American craton to the east. The enclosing Cerro Toro flysch represents overbank and pelagic deposits. Most of the conglomerates have features developed by tractive transport; they are moderately well sorted, imbricated, and have both horizontal and inclined internal stratification. Large-scale dunes up to 4 m high are present. Fabrics and bedding styles considered typical of gravity flows are less common. Features produced by traction transport apparently were developed in the gravel by rolling, sliding, and saltation as the bedload component of highly turbulent, probably low-density, turbidity currents flowing in a confined channel.

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.

Cambrian Tropical Storm Waves in Wisconsin

Sizes and shapes of quartzite boulders in Cambrian strata surrounding ancient islands at Baraboo, Wisconsin, allow estimation of the magnitude of Cambrian ocean waves using results from wave tank and flume experiments, as well as observations of very coarse stream gravel. Breakers at least 7 to 8 m high were required to tumble and move the largest well-rounded boulders (1.5 m diam) interstratified with Cambrian sandstone. Storm generation of waves of such magnitude is consistent with independent paleomagnetic evidence that suggests that Wisconsin lay near the equator 500 m.y. ago, at which latitude intense tropical storms would be expected.

Sedimentology, stratigraphy, and extinctions during the Cretaceous-Paleogene transition at Bug Creek, Montana

Bug Creek Valley, the source of an unusual and controversial Cretaceous-Paleogene coincidence of mammals, dinosaurs, pollen, and indium, exemplifies the importance of depositional process in the reconstruction of evolutionary events. Five sedimentary facies are recognized at Bug Creek: a cross-stratified sandstone, a green and purple siltstone, a lateral accretionary sandstone, a coal, and a variegated siltstone. Repeated fluvial channeling restricts the accuracy of lateral correlations, and the relationship of the fossil assemblage to the presumed Cretaceous-Paleogene boundary cannot be established. Sedimentologically, the Cretaceous-Paleogene transition is represented here by Cretaceous meandering channels that gave way initially to Paleogene swamp deposition.

Episodic event deposits versus stratigraphic sequences—shall the twain never meet?

Abstract In the early half of the 20th century, questions were raised about different rates of deposition reflected in the stratigraphic record and of small gaps or diastems. Even more discussion centered upon rhythms and cycles of deposition—at one extreme were glacial varves and at the other were Carboniferous cyclothems. After World War II, interest in stratigraphic cycles declined. Then the turbidity current revolution stimulated interest in event deposits, which interest has surged again recently with a focus upon storm deposits. Meanwhile, the recent dramatic growth of sequence stratigraphy has rekindled interest in both cyclicity and eustasy. The two themes—events and cycles—should be better integrated, for there is considerable confusion about the interpretation of high-frequency sequences. There is also a need to reconcile the current fad for Milankovitch-related sedimentary cycles versus more or less random event deposits. The most familiar event deposits are turbidites in deep-water and tempestites in shallow-water environments. More subtle are the diastems, which include non-depositional surfaces as well as scoured surfaces. Other processes that can produce event deposits include avalanches and tsunamis. Potentially, any type of event deposit could be mistaken for a sequence boundary. For example, submarine megabreccias could be formed either by a seismic event unrelated to any sea level change, or by slope failure resulting from a eustatic fall associated with a sequence boundary. To surmount the intellectual barrier to alternate interpretations requires careful attention to processes, time resolution, and objective tests for periodicity.

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.

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.

Cambrian Shoreline Deposits in Northern Michigan

ABSTRACT Shoreline deposits have been recognized rarely in pure quartz arenites that were deposited before the advent of land vegetation. The Late Cambrian Munising Formation of northern Michigan is an exception. Both conglomerate and shale are relatively more common here than in most similar cratonic sandstones, apparently because of an important fluvial influence. A basal conglomerate member is interpreted as a low-sinuosity (braided) fluvial deposit. The overlying Chapel Rock Member has unfossiliferous, large-scale cross bedded sandstones with minor but widespread conglomerate interpreted as braided fan-delta distributary deposits and associated horizontally-stratified sandstone with wave ripples, polygonally-cracked shales, and trace fossils, interpreted as interdistributary deposits. Both of these latter facies are punctuated by unsorted, unstratified channelized conglomerate and intraclast breccia, which are interpreted as river-flood avulsion deposits. Although fluvially-dominated, the depositional system also had wave and probable tidal influences as well as local eolian reworking of fluvial sands. Thus a shoreline setting adjacent to highlands centered over the northern Michigan-Wisconsin border is indicated. The overlying Miners Castle Member is interpreted as transgressive marine deposits formed as nearby highlands became mostly submerged. The lower strata are sandstones with medium-scale trough cross bedding; thin flaser-like shale drapes accumulated along many cross laminae and over rippled beds. These are interpreted as prodeltaic. The upper strata have little shale but much bioturbation and some skeletal fossils, and are interpreted as more distal shelf deposits. The rather unusual association in the Munising Formation of conglomerate and shale with cross-bedded quartz arenite, the particular distribution of trace and skeletal fossils, and of mud crocks and wave ripples, allow an environmental reconstruction that may provide valuable analogies for other, hitherto-unrecognized, pre-vegetation shoreline deposits.