The shales exhibit well developed, normally graded lamina. Sediment grain sizes range from silt and sand as the base of the lamina up to silt and clay at the top. Computer-based optical scanning of prepared shale samples was undertaken so that the lamina thicknesses could be extracted automatically and then used for thickness-series analysis. The sands and silts tend to very light in color because they consist of well-sorted, quartz-rich sands.
Conversely, the silt and clay that cap the lamina tend to be dark because of small grain size; such fine-grained layers also contain large amounts of macerated and coalified plant material. Grain size and sedimentological differences within the laminae can be detected by optical properties and this facilitates automated image processing. Extraction of laminae thicknesses and harmonic analysis of laminae-thickness series can then be used to test the reasonability of specific environmental processes, such as tidal forcing, that were operative during the formation of these shales.
Throughout the Late Carboniferous, wet paleoclimates associated with these coals lead to valley incision during sea-level lowstand when large tropical rivers downcut older sediments deposited during previous sea-level highstands. During subsequent rise of sea level, these fluvial valleys were flooded and converted to estuaries where tidal ranges and sedimentation rates were significantly amplified.
Based on modern analogs and interpretations of many examples of Carboniferous rhythmites, the localized depositional rates in these settings are exceptionally high. The estuaries became sediment sinks, trapping sediment that is pumped in from both fluvial and marine sources. As a result, sedimentation readily keeps pace with rising baselevel. Extensive intertidal flats and shallow subtidal flats are created and prograde over the valley-confined mires. Thick tidal cycles and upright trees (some with attached foliage) record rapid burial of mires.
This model is supported with examples of roof facies from the Westphalian B-C of the Eastern Interior Basin, and the Stephanian of the Western Interior Basin. In these areas facies within each cycle range from well-developed, extensive paleosols and coals, to widespread marine shales or limestones. Variations in both sea level and climate resulted in a complex history of valley fill during which coals could be developed at any time (except during widespread marine flooding). Minable, low-sulfur and low-ash coals occur, but the coals are relatively thin and discontinuous. Conversely, the Westphalian D coals are very widespread and significant peat accumulations were not confined to paleovalleys. Nonetheless, the lowest sulfur coals are related to rapid deposition of roof facies that occurred within the paleovalley whereas high-sulfur coals were formed in areas of lower sedimentation rate of roof facies that occurred beyond the confines of the paleovalley.
Abstract: The Douglas Group (Stephanian) of eastern Kansas contains several paleovalleys that were eroded during falling sea level and filled during lowstands and subsequent transgressions. One paleovalley exhibits 34 m of incision, is approximately 32 km in width, and can be laterally traced along outcrop and into the subsurface to the south for approximately 140 km. A fluvial to estuarine to marine facies mosaic can be delineated both laterally, from north to south, as well as within individual vertical sections.
Paleovalleys were filled with a fining upward succession; the lowest facies is cross-bedded conglomerate and sandstone. The conglomerate contains clasts and fossils eroded from older units exposed within the paleovalley. Sandstone beds exhibit large scale (up to 1 m thick) trough and tabular-planar cross beds. Paleocurrent directions are generally southwest and indicate deposition via large-scale fluvial systems that were constrained within the paleovalleys.
Overlying the fluvial sandstone is a diverse suite of lithofacies including planar-bedded sandstones and siltstones, heterolithic facies, sheet-like sandstone, bioturbated sandstones, and marine facies. The planar-bedded sandstones and siltstones can exhibit neap-spring tidal cycles which were formed in high-intertidal settings. Heterolithic facies are typically laminated and contain pinstripe laminations, starved ripples, and well-developed tidal cycles (cyclical tidal rhythmites). Neap-spring tidal cycles are common and range from 1 cm in thickness in heterolithic facies to as much as 1 m in thickness in planar-bedded siltstones. An interpretation invoking very high localized depositional rates is substantiated by the presence of buried upright trees, some of which have attached foliage. Tidal rhythmites are well developed in siliciclastic facies immediately overlying coals. The heterolithic and silty rhythmites were apparently developed within the estuarine turbidity maximum where high turbidity and locally high depositional rates resulted from estuarine circulation patterns and tidal amplification.
The sheet-like sandstone bodies are dominated by small-scale trough crossbedding and ripple- and planar laminations. Paleocurrents are bimodal to the southwest and northeast, reflecting ebb- and flood-tidal currents. Features such as flat-topped ripples, rain-drop imprints, and tetrapod trackways indicate deposition within the intertidal zone.
Estuarine to marine sequences contain progressively higher diversities of biogenic structures. "Flaggy" bioturbated sandstones indicate significant marine influences. These sandstones are capped by widespread marine shales and limestones that extend far beyond the limits of the paleovalleys. Shales can be extensively bioturbated, lack laminations, and locally contain marine body fossils. Limestones form widespread lithostratigraphic markers and contain abundant marine fossils such as bivalves, fusulinids, brachiopods, crinoids, and bryozoans. Some of the limestones consist of shelly lags which indicate the development of transgressive surfaces of erosion.
There are two major sequences developed within the Douglas Group. The sequence boundaries can be placed at the contact between incised fluvial sandstones and underlying, commonly marine, strata. The fluvial and estuarine facies were deposited during lowstand and subsequent sea-level rise. The highstand system includes marine shales and limestones which were erosionally incised during subsequent fall in sea level.
Abstract: The oldest known tidal rhythmites, identified in the Big Cottonwood Formation, Utah, are Late to Middle Proterozoic in age (800 Ma to 1.0 Ga), ~250 to 400 m.y. older than the previously oldest tidal rhythmites. Four tidally forced cycles and one nontidal (seasonal) cycle control lamina thickness patterns. All of these cycles are recognized in outcrop and core and include cycles associated with daily, semimonthly (synodic), monthly (anomalistic), semiyearly, and yearly (seasonal) events. These features form the oldest geological record of lunar-solar tidal forcing and show that the middle to late Precambrian lunar- and solar-generated tides behaved in a manner very similar to that of today. The analysis also suggests that the Big Cottonwood Formation may have undergone a seasonal climate.
Abstract: The Bear Gulch Beds of the Heath Formation are well known for their diverse and well-preserved assemblage of fish, arthropods, and soft-bodied animals (the constitute a Lagerstatte). The Bear Gulch is a lens of lithographic limestone (approximately 12 km in lateral extent and up to 30 m thick) surrounded by black, platy shale. The lens is composed primarily of alternating massive beds (up to 25 cm thick) and argillaceous platy beds (up to 30 cm thick). Platy and massive beds are both composed primarily of normally graded laminations (1-15 mm thick). Laminations typically have sharp bases and grade upward from microspar to micrite. Lateral continuity of individual beds (at least 1 km) and laminations (at least 500 m), lack of evidence of bottom currents, and paucity of erosional features all suggest a quiet-water environment. Fossils are generally rare in the Bear Gulch Beds. The most common fossils in most beds are cephalopods, shrimp, fish, and soft-bodied organisms.
Abstract: Pennsylvanian-age, small-scale tidal bundle sequences discovered on the eastern margin of the Illinois Basin record years of continuous daily tidal deposition, and have been used to interpret the astronomical factors influencing deposition. Because they also provide an absolute time scale of daily, monthly, semiyearly, and yearly resolution, these records can be used to identify nontidal factors of short duration. Deviations from the semiyearly and yearly sedimentation patterns predicted from pure tidal theory were observed in some of these tidal bundle sequences, and we interpret them to be the result of seasonal rainfall variations that affected sediment discharge. The possibility that seasonal climates existed in humid tropical paleoclimatic zones during the Pennsylvanian has been previously discussed, but analysis of the semi-yearly and yearly depositional patterns in tidal rhythmites of the Illinois Basin provides direct evidence for their occurrence.