Hyperpycnal flow deposition and sequence stratigraphy of a cretaceous near-shore mudstone unit - the Skull Creek Shale Formation, Colorado, USA.

摘要

The middle shale member of the Cretaceous Dakota Group, the Skull Creek Formation (Fm.), is a proximal mudstone unit deposited in a delta-fed system within the Cretaceous Interior Seaway. Based on lithology, sedimentary structures, bioturbation intensity, and TOC values, the Skull Creek Fm. was divided into twelve facies which were grouped into five facies associations (FAs). All facies and FAs record deposition along proximal to distal regions within an ancient mud-dominated continental shelf. Sediment transport and deposition were strongly influenced by both river flooding and storm events that generated hyperpycnal flows. Sediments in the Skull Creek Fm. were transported as bed load by a combination of wave and current energy above storm wave-base (FA 5, FA 4, and parts of FA 5) or exclusively by currents as the flow traveled below storm wave-base (parts of FA 3, FA 2, and FA 1). Mud-rich siltstones and sandstones (FA 4) and bioturbated siltstones and sandstones (FA 5) are the most proximal FAs and were deposited above storm wave-base while ripple- to planar-laminated siltstones and mudstones (FA 3) were interpreted to be "medial" expressions of hyperpycnal flows due to the low occurrence of wave generated structures. The most distal FA's include massive, bioturbated mudstones (FA 1) that were deposited in a sediment starved area, most likely located laterally or in a more distal location to the laminated mudstones (FA 2). Laminated mudstones (FA 2) were transported as bed load within turbulent flows below storm wave-base. Fining-upward parasequences containing fine-grained FAs (FA 1 and FA 2) represent decreasing energy conditions and a rise in sea level, while stacked successions of more proximal units (FA 3, FA 4, and FA 5) are indicative of high energy levels and lower sea levels. Based on the stacking patterns of the five FAs, the Skull Creek Fm. was divided into five laterally traceable stratigraphic intervals. Transgressive system tracts are observed in interval 1 and interval 2, which are fining-upward successions composed of FA 5, FA 4 at the base and FA 1, FA 2, and FA 3 at the top. Interval 3 contains mainly HCS sandstones and siltstones which thicken vertically and it represents the only highstand system tract (HST) within the Skull Creek Fm. A regressive sequence is preserved in interval 4 which is composed of the most proximal FAs (mainly FA 5) and is abruptly overlain by interval 5, which is a fining-upward succession, and represents a final transgressive system tract within the Skull Creek Fm. TOC (total organic carbon) content is highest, and contains the most oil-prone organic matter, within interval 2 which is composed of mainly laminated mudstones (FA 2). The high TOC values within FA 2 are due to low levels of bioturbation and low levels of sediment accumulation, whereas in other FAs moderate to high bioturbation intensities and high sedimentation rates decrease the likelihood of preserving organic matter. "Sweet spots" within other similar, proximal mudstone units that were deposited as hyperpycnal flows are therefore predicted to be found in distal deposits, where there was limited sediment reworking by organisms.