Tabular Geometry Research Articles

Cross‐strata and bedforms of probable transitional dune to upper‐stage plane‐bed origin from a Late Precambrian fluvial sandstone, northern Norway.

ABSTRACTA Late Precambrian fluvial sandstone sequence in northern Norway is dominated by large‐scale cross‐sets that show either lenticular or tabular geometries in the streamwise sections. The lenticular sets interdigitate and in places show nearly symmetrical formsets. The tabular sets are in places solitary, but are mainly grouped in cosets. In both cross‐set types, the cross‐strata range from concave‐up to sigmoidal in shape, with the latter variety comprising subhorizontal to gently inclined topset strata (with parting lineation) that merge uninterruptedly downflow into the steeper (10–2°) foresets. Within the cross‐sets the geometry and dip azimuths of the foresets are conspicuously consistent, although the concave‐up and sigmoidal strata commonly alternate downcurrent.The cross‐strata characteristics suggest flood stage deposition from relatively high velocity steady currents heavily laden with suspended sand. Both cross‐set types are interpreted as representing bedforms generated by flow in the dune to upper‐stage plane‐bed transition. The lenticular cross‐sets probably represent periodic dunes, but it is far less clear whether the long bedforms represented by the tabular sets should be classified as dunes, or rather as solitary to quasi‐periodic bars.

Lower Eocene alluvial paleosols (Willwood Formation, Northwest Wyoming, U.S.A.) and their significance for paleoecology, paleoclimatology, and basin analysis

The lower Eocene Willwood Formation of northwest Wyoming is a 700 m thick accumulation of alluvial floodplain and channel mudstones and sandstones, nearly all of which show paleopedogenic modifications. Pedogenesis of Willwood sandstones is indicated by taproot and vertebrate and invertebrate bioturbation, early local cementation by calcium carbonate, and thin illuviation cutans on clastic grains. Pedogenesis in Willwood mudstones is indicated by plant bioturbation, insect and other invertebrate burrow casts and lebensspuren; free iron, aluminum, and manganese mobilization, including hydromorphic gleying; sesquioxide and calcareous glaebule formation in lower parts of the solum; presence of clay-rich and organic carbon-rich zones; and well differentiated epipedons and albic and spodic horizons. Probable A horizons are also locally well developed. Occurrence of variegated paleosol units in thick floodplain mudstone deposits and their association with thin, lenticular, and unconnected fluvial sandstones in the Willwood Formation of the central and southeast Bighorn Basin suggest that these soils formed during times of rapid sediment accumulation. The tabular geometry and lateral persistence of soil units as well as the absence of catenization indicate that Willwood floodplains were broad and essentially featureless. All Willwood paleosols were developed on alluvial parent materials and are complex in that B horizons of younger paleosols were commonly superimposed upon and mask properties of suspected A and B horizons of the next older paleosols. The soils appear to be wet varieties of the Spodosol and Entisol groups (aquods and ferrods, and aquents, respectively), though thick, superposed and less mottled red, purple, and yellow paleosols resemble some ultisols. Most Willwood paleosols resemble warm temperate to subtropical alluvial soils that form today under alternating wet and dry conditions and (or) fluctuating water tables. The up-section decrease in frequency of gley mottles, increase in numerical proportion and thickness of red versus orange coloration, and increase in abundance of calcrete glaebules indicate better drained soils and probably drier climate in late Willwood time. This drying is believed to be related to creation of rain shadows and spacing of rainfall (but not necessarily decrease in absolute rainfall) due to progressive tectonic structural elevation of the mountainous margins of the Bighorn Basin.

Palaeoaplysina (Hydrozoan?) Carbonate Buildups from Upper Paleozoic of Idaho

The plate-shaped calcareous hydrozoan? Palaeoaplysina Krotov is the main fossil in Late Pennsylvanian and Early Permian carbonate buildups in the Lemhi Range and Arco Hills of east-central Idaho. Palaeoaplysina has an internal subparallel canal system imbedded in cellular tissue, and mamelon-like surface protuberances. Other fossils in the buildups include the calcareous phylloid alga Eugonophyllum, other unidentified phylloid algae, crustose bryozoans, and less common gastropods, crinoids, and brachiopods. Buildup sediment accumulated primarily by the baffling and entrapment of loose carbonate pellets, lime mud, and skeletal fragments in protected pockets and hollows between Palaeoaplysina and algal plates. There is a continuum between tabular and lenticular buildup geometries. Broad tabular buildups are typically 2 to 10 m thick and as much as 15 km wide. The basal surfaces of the tabular buildups conform to previous sea-floor topography, whereas upper surfaces are relatively flat and planar, suggesting that the buildups were limited in upward growth because they formed in the uppermost subtidal zone. The abundance of porcelaneous tubular foraminifers (resembling apterrinellids and Hedraites), which encrusted some unpreserved organisms, suggests that some buildups were carpeted by unfossilized vegetation such as noncalcareous algae. Strongly lenticular buildups are typically 2 to 16 m thick and a few tens to hundreds of meters wide. Most were large enough to modify depositional conditions nearby, resulting in fore-mound and back-mound beds. All of the lenticular buildups, and some of the tabular ones, stood topographically above the adjacent sea floor while they formed. Some buildups are stacked one above another. Others are in offlapping sequences which also include lenticular beds of crinoidal-bryozoan lime wackestone and grainstone.