Keith B. Miller

Lower Permian (Wolfcampian) Paleosol-Bearing Cycles of the U.S. Midcontinent: Evidence of Climatic Cyclicity

Lower Permian sedimentary cycles of the North American Midcontinent consist predominantly of very shallow marine and paralic facies and well-developed stacked paleosol profiles. Although recording glacio-eustatic sea level fluctuations, these cycles also contain evidence of cyclic climate change. This evidence includes the repeated carbonate-to-clastic facies pattern observed for meter-scale cycles, and the regionally consistent change from calcic to vertic paleosols within the variegated mudstones of most cyclothems. Climates are interpreted to have fluctuated from arid or semiarid conditions to seasonally wet/dry conditions during the course of a single cyclothem. Furthermore, within the Midcontinent, drier conditions appear to have characterized times of sea-level rise and highstand, whereas wetter more seasonal conditions characterized times of sea-level fall and lowstand. This relationship is interpreted to have resulted from variations in the intensity of a Pangean monsoon generated by glacial-interglacial cycles.

The role of climate in stratigraphic patterns exhibited by late Palaeozoic rocks exposed in Kansas

Abstract Late Palaeozoic “cyclothems” of the midcontinental U.S. cannot be represented by a single ideal facies sequence. Rather, they encompass a wide range of cycle types depending on their stratigraphic position. The exposed Late Palaeozoic (Pennsylvanian and Permian) rocks of Kansas, as compiled by Zeller (1968), can be divided into nine major lithofacies. These lithofacies were ordered based on their times of greatest abundance through the section. From oldest to youngest, they attain their peak abundance in the following order: (1) coal; (2) gray mudrocks and sandstones; (3) black mudrocks; (4) limestones; (5) cherty limestones; (6) fossiliferous mudrocks; (7) variegated mudrocks; (8) evaporites; and (9) variegated dolomitic siliciclastics. Climatically, these nine lithofacies suggest a change from generally wetter, at the base, to generally drier, at the top of the section. This climatic trend was probably generated by a complex interaction of factors primarily driven by global tectonics. Climatic changes can be used to explain the five generally recognized types of lithofacies “cyclothems” in Kansas, and thus are a potential explanation for the changing stratigraphic patterns of the entire Late Palaeozoic time interval of Kansas, and perhaps elsewhere.

Taphonomy and depositional dynamics of Devonian shell-rich mudstones

Abstract Fossil preservation (e.g. articulation of multi-element skeletons) provides important insights into depositional dynamics. Detailed taphonomic analysis of marine mudstones and associated shell beds from the Middle Devonian of western New York indicates that these beds record primarily episodic sediment deposition of short duration. We propose a storm winnowing, mud blanketing model to explain the variety of shell and mudstone beds observed, and to explain apparent onshore-offshore gradients of shell bed characteristics. Shell beds themselves appear to have accumulated during relatively long spans of time ranging up to perhaps thousands of years. Most are internally complex and exhibit evidence for multiple events of burial and exhumation by winnowing. Shell accumulation facilitated colonization of the seafloor by hard substrate-encrusting organisms; these contributed to increasingly complex communities that were ultimately destroyed and permanently blanketed by thick mud layers. The sparsely fossiliferous mudstones, which compose up to 85% of the Hamilton stratigraphic section, mainly record very abrupt inputs of sediment; in some instances taphonomy of enclosed fossils indicates deposition of layers up to several centimeters thick (prior to compaction) within hours to a few days. In turn, some sequences of shell and mudstone beds were deposited within the life span of single colonial organisms, and thus also record only relatively short spans of time overall. This reinforces our conclusions that most time in mudstone sequences must be represented by discontinuities at sharp surfaces or thin condensed beds that bound more or less continuous bundles of shell and mudstone beds. Again, these often subtle discontinuities can be recognized on the basis of taphonomic criteria. Many complex shell beds as well as simple shell beds can be traced along depositional strike for tens of kilometers. Condensed shell beds only a few centimeters thick can commonly be correlated for 100 to 200 km along and across depositional strike. The correlatable nature of shell beds apparently reflects the temporal contrast between these beds and enclosing muds; on a regional scale, shell beds represent integrated accumulations that were deposited over spans of tens to thousands of years. Conversely, the mud layers that blanketed and terminated shell bed accumulations may represent very rapid depositional events (spanning hours to days), which are locally isochronous. Thus, the study of taphonomy of shell beds reveals a good deal of detail about their depositional history; these studies imply that the stratigraphic record is even more episodic and discotinuous than commonly recognized. Episodic deposition implies that while the stratigraphic record of epeiric seas may be adequate for examining details of community succession and paleontology, it probably can not generally be used for microevolutionary studies other than examination of patterns of stasis.

Identification of Sequence Boundaries Within Cyclic Strata of the Lower Permian of Kansas, Usa: Problems and Alternatives

In the past, midcontinent cyclothems have been defined on the basis of cyclic facies patterns, rather than discontinuity surfaces. Sequence stratigraphic concepts, although offering a potential means of recognizing and correlating cycle boundaries, have proven difficult to apply to the lower Permian cyclothems of the midcontinent. The cycles of the lower Permian of Kansas are dominated by very shallow marine and paralic facies with a multitude of subaerial exposure surfaces ranging from desiccation cracks to well‐developed paleosols. Meter‐scale shallowing‐upward cycles (para‐sequences) are prominent, and most are capped by subaerial exposure surfaces. Furthermore, incised valleys and valley fills are conspicuously absent. Sequence boundaries are usually equated with surfaces recording maximum relative sealevel lowstand. Within a stacked series of paleosols this would presumably coincide with the most mature paleosol profile. However, relative maturity is difficult to assess if the paleosols were developed under differing topographic or climatic conditions and/or truncated by subsequent transgression. Alternatively, cyclothem boundaries could be defined at the prominent marine flooding surfaces that overlie these paleosol‐bearing intervals. These surfaces are overlain by fossiliferous marine limestones and are typically associated with intraclastic beds. They are equivalent to the transgressive surfaces of sequence terminology and provide more easily recognized and correlated boundaries. The use of transgressive surfaces rather than lowstand unconformities may provide a more practical application of the sequence model to midcontinent Permian cycles.