Abstract

The Oligocene-Miocene West Crocker Formation is a major basin-floor turbidite-fan complex that was deposited in an accretionary foredeep basin. The system is at least 1 km (0.6 mi) thick and more than 25,000 km² (9650 mi²) in extent, rivaling other world-class modern and ancient turbidite systems. It comprises several stacked, sand-rich, composite megasequences constructed of numerous higher frequency sequences.

Vertical facies successions have been logged from several separate outcrop exposures, each up to ~300 m (~980 ft) thick, which are located along a 100-km (60-mi)-long structural strike section. The stratigraphic succession records a basin-floor turbidite system constructed of two end-member depositional elements: (1) mixed sand-rich to mud-rich leveed channels, which may serve as updip feeder or bypass systems, and (2) sand-rich leveed channels transitional to braid-plain distributary-channel lobes/sheets (frontal splays).

The leveed-channel systems (30–60 m [100–200 ft] thick) fine and thin upward, comprising aggradation and lateral migration of channel-axis, channel-margin, and proximal–distal-levee deposits. The braid-plain distributary-channel-lobe complexes (5–20 m [16–66 ft] thick) thicken and coarsen upwards and are associated with sand-rich sheet elements. We propose a depositional model in which a confined, updip leveed–channel system passes downdip to a more varied system of lower relief, possibly lower sinuosity, leveed channels to braided-channel sheets.

Facies successions are interpreted as the result of channel-levee migration of sand-rich or mixed sand-mud systems, stacked multistory channels, and low-relief, anastomosing, distributary-channel-lobe elements (single channel-fill successions ranging from 10 to 60 m [33–200 ft] thick). Typically they comprise >80% sandstone and were deposited by composite, amalgamated flows and organized into multiple genetic-fill sequences. Sand-rich, leveed-channel-axis and terminal-braid-plain-channel facies consist of megabeds (2–3.5 m [7–12 ft] thick) of mainly moderately to poorly sorted, medium- to very coarse-grained sandstone. The most common internal structures are massive beds, diffuse wavy to parallel layering, and dewatering structures. In places, they also display planar, grain-sorted, parallel-layered sheet bed-forms, and cross-stratification. The channel-margin facies and braid-plain-sheet facies consists of mainly medium- to coarse-grained sandstone with massive to diffuse horizontal lamination, flow-stripped current-ripple cross-lamination, and thin debrite caps. Interchannel braid-plain bars are constructed of shingled, lenticular bedforms and more common debrite beds.

The megabeds may provide an important genetic link between the more confined, updip, levee-channel-axis, massive sandstones and the downdip, thickening-upward, braided-channel, massive sandstone plugs. The individual, 5–30-m (16–100-ft)-thick genetic sequences are organized into larger scale, 50–100 m (165–330 ft) megasequences, which display an overall succession of thickening- or thinning-upward stacking patterns. The megasequences are interpreted to be high-order composite depositional sequences, which are capped by regionally extensive mud-dominated intervals. The latter mark major changes in sediment supply and fan evolution and, in some cases, enable intrabasinal correlation.