SEQUENCE STRATIGRAPHY AND LAGERSTÄTTEN IN THE MIDDLE CAMBRIAN, GREAT BASIN, UTAH

Ongoing study of high-resolution sequence stratigraphy of the Middle Cambrian Wheeler and Marjum formations in the Drum Mountains and House Range, west-central Utah has revealed recurrent patterns of conditions resulting in the preservation of various types of fossil Lagerstätten. The mixed siliciclastic–carbonate units are divisible into cyclic 3 to 20 m thick intervals resembling high-order depositional sequences/cycles. These sequences commence with compact, oncolitic to oolitic pack- and grainstones with sharply-defined upper contacts inferred to record drowning discontinuities; these beds are interpreted as early transgressive systems tracts (TSTs). Overlying intervals of calcareous shale and thin-bedded wacke- to packstones with abundant, mainly disarticulated polymerid and agnostid trilobite remains, are inferred to represent late TSTs. These are followed by pinkish gray limestones, rich in sponge spicules and with hardgrounds that record maximum flooding zones and are overlain by relatively thick, early highstand (HST) intervals of black, fissile to dark gray, platy, calcareous shale with abundant articulated agnostid trilobites and, in some instances, the polymerid Elrathia. Finally, these beds pass upward into interbedded shale and thin, buff-weathering calcisiltites (probable turbidites/distal storm beds), and burrow-mottled to nodular limestones, recording late HST to falling stage (FFST) carbonate shedding.

Concentration Lagerstätten , represented by spicular and fragmental trilobite hash beds, are associated with the sediment-starved conditions of the TST, while conservation Lagerstätten occur primarily in the overlying sparsely fossiliferous highstand shales. Obrution Lagerstätten of articulated trilobites and eocrinoids were preferentially preserved by episodic mud tempestites/turbidites on more oxic seafloors during later highstand; the high sediment influx typical of FSSTs resulted in sparsely fossiliferous strata. Conversely, the combination of lower dysoxic-anoxic conditions, abundant organic detritus, and relatively rapid episodic influx of detrital sediment favored repeated burial and preservation of organic remains, primarily algae, but in rare instances, including soft-bodied animal remains, during the early HST portions of cycles