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Outcrop and Subsurface Characteristics of Tidally Modified Shelf Edge Delta and Upper Slope Deposits of the Blair Formation, Rock Springs Uplift, Wyoming

Abstract

The Campanian Blair Fm (Blair) outcrops along the Rock Springs Uplift, WY (350- 500m

thick) and is interpreted to be an isolated shallow marine sandbody (ISMS). ISMS are pseudo-

linear, elongate sandbodies, partially to completely encased in mudrock, isolated from their

coeval distributary systems, and commonly act as hydrocarbon reservoirs (e.g. Tocito Fm); yet

their origins are debated. For example, the Blair has been interpreted as both tidally-influenced to

wave-modified delta deposits, and deep-water (~300m) slope fan deposit. Our integrated

sedimentologic, ichnologic, and architectural outcrop-subsurface study of the Blair including 8

composite measured sections and 150+ well logs refines interpretations of the depositional

system.

Our results indicate that the Blair is a southeast-thickening shelf-edge delta to slope deposit,

transitioning from a progrational lower (L) Blair, to a retrogradational middle (M) Blair, and

progradational upper (U) Blair. The L Blair (20-38m thick) comprises stacked, sand-rich

coarsening upward packages up to 10m thick with sandstones (sst.) containing modified current

ripples, sigmoids, trough cross-stratification (CS), mud rip-up clasts, mud drapes, and rare

herringbone cross-strata intercalated with finer-grained flaser, wavy, and lenticular (FWL)

bedded siltstone. We interpret the L Blair as tidally-influenced prodelta to delta front deposits,

exhibiting internal sequences of upward decreasing gamma ray (GR) well log character. The L-

M Blair transition is defined by a regionally extensive, (40-100m thick) siltstone-mudstone

interval dominated by FWL bedding, interpreted as distal shelf to upper slope deposits. The M

Blair is dominated by high GR, but punctuated by blocky or upward decreasing GR intervals,

some representing large-scale (2-5m) slumped/folded intervals and incisions (2-12m deep)

interpreted as a slope channel - turbidite complex based on: massive/convolute bedded sst.,

tabular (~5cm) bouma sequences, and ripple and trough CS Sst. The M-U Blair transition is

defined by reintroduction of sst. intervals interpreted as the transition from slope to shelf-delta

deposits. The U Blair contains coarsening upward packages with hummocky CS interpreted as

wave-modified prodelta and delta front deposits exhibited by stacks of upward decreasing GR.

We interpret the Blair regressions-transgressions to be controlled by changing basin subsidence

rates with sediment supply remaining high throughout Blair deposition.