Geophysical and geological signatures of relative sea level change in the upper Wheeler Formation, Drum Mountains, West-Central Utah: A perspective into exceptional preservation of fossils

摘要

The Middle Cambrian Wheeler Formation in Utah is renowned for its exceptionally preserved fossils. Herein, geophysical techniques (gamma ray spectrometry and magnetic susceptibility), carbonate analyses, lithofacies, and fossils are used to characterize limestones and calcareous mudrocks of the upper portion of the middle Wheeler Formation and the upper Wheeler Formation in the Drum Mountains, West-Central Utah. This succession was deposited on a mixed carbonate-siliciclastic ramp within the House Range Embayment, an eastward incursion of deeper water, created and bounded on the southeast by a normal fault. Geophysical data clearly reveal variations in carbonate versus clay content. Observed patterns among outcrop geophysical data, lithofacies, and fossils are interpreted within the contexts of relative sea level changes and a sequence stratigraphic model. Above a basal interval of rhythmically-bedded limestones interpreted to have been deposited on the middle part of the ramp during a period of relatively low sea level, lithofacies, rising gamma ray and rising magnetic susceptibility track a transgression, which culminates in a maximum flooding surface (MFS). This MFS is near the base of a "hot zone": an similar to 6-m-thick interval of exceptional preservation (Konservat-Lagerstatte), highest magnetic susceptibility, highest gamma ray values, highest clay content, and with a bulk carbonate content of similar to 4-20%. The basal layer of this zone contains abundant, fully articulated specimens of agnostids and tiny polymerid trilobites. Exceptionally preserved fossils, such as non-trilobite arthropods, priapulid and annelid worms, hyolithids, phyllocarid arthropods with soft parts, algae, and fragile sponges, are much more abundant within this hot zone than in other strata of the succession. These specimens are preserved in very thin-bedded to laminated mudrock, only rarely interrupted by macroscopic signs of bioturbation. Stratigraphically above the hot zone is a thick unit of mudrock in which both gamma ray and magnetic susceptibility decrease, gradually at first and then precipitously to very low levels; in concert, few soft-bodied specimens have been recovered from this upper interval. The gradual decrease in geophysical responses is interpreted to be the result of gradual shallowing during highstand, caused mainly by progradation of the carbonate factory; it was followed by an abrupt regression. Mudrocks are capped by burrow-mottled grainstone and stromatolitic boundstone, representing a lowstand or the earliest transgressive phase of the next cycle. These observations lead to the following conclusions: (1) the upper Wheeler Formation represents most of a third or fourth-order cycle of relative sea level change on a mixed carbonate-siliciclastic ramp, rather than a period of shallow-water lagoonal sedimentation as previously proposed; (2) superimposed on this overall cycle are several (perhaps many) higher-order fluctuations in relative sea level; (3) when the present results are coupled with results of earlier workers, who interpreted the lower and middle Wheeler Formation as a major transgression and regression, the Wheeler Formation of the Drum Mountains involves two major sea level cycles, rather than one; (4) the Konservat-Lagerstatte was preserved in the deepest-water, early-highstand portion of the upper Wheeler sequence; and (5) these results support earlier hypotheses that Konservat-Lagerstatten in mixed carbonate-siliciclastic successions are most likely to occur during late transgression to early highstand, given that low energy,anoxic conditions prevailed. (C) 2009 Elsevier B.V. All rights reserved.