Preservation of laminated shale in ancient clinoforms; comparison to modern subaqueous deltas

Abstract

In Phanerozoic fine-grained marine sedimentary deposits, preservation of physical lamination is commonly presumed to record anoxic benthic conditions. Lamination may also be preserved, however, in oxygen-rich settings where sediment influx is rapid and, as a consequence, high levels of turbidity and fluid substrates inhibit colonization by benthic organisms. Differentiation between these two situations is critical for accurate paleoenvironmental interpretations. The recognition of clinoforms, which are common in ancient fine-grained successions, is a key step in making this distinction. In the Tropic Shale and correlative Tununk Member of the Mancos Shale of southern Utah, clinoforms are preserved in two superposed depositional sequences, the fourth and fifth sequences in the succession. The geometry of the clinoforms indicates that foreset deposits accumulated relatively rapidly and bottomset deposits more slowly. In sequence 4, patterns of preservation of lamination in clinoforms resemble those in modern subaqueous deltas. These ancient clinoforms evidently prograded into oxygenated waters, and preservation of lamination was primarily controlled by the rate of sediment accumulation. In sequence 5, however, the oxygen level apparently controlled the preservation of lamination.