Anatomy of a Cretaceous Tide-Influenced Subaqueous Delta: The O'Brien Spring Member, Haystack Mountains Formation, S. Wyoming

Uroza, Carlos A.; Steel, Ron

Shallow-marine sandstone bodies encased or semi-encased within marine shales, on the most distal fringes of the Campanian clastic wedges of the Cretaceous Western Interior Seaway, have long been depositional enigmas. Here, we present an analysis of one of such sandstone bodies, the O'Brien Spring Member of the Haystack Mountains Formation (South Central Wyoming) and suggest that its anatomy sheds light on the broader problem of the distal tips of the clastic wedges.

Based on outcrop facies, paleocurrent readings, and outcrop photography we interpret the O'Brien Spring Member as a large tide-dominated delta system because of its delta-scale clinoforms (30-50m), upward coarsening and thickening of beds within the clinoform sets, mudstone layers as dune toe-sets and extensive thin muds between rippled and cross stratified sandstone sets, bi-directional paleocurrents in places, and predominance of cross strata representing 2-D and 3-D dunes on the upper parts of the main delta body. The lobes of the O'Brien Spring system were entirely subaqueous, as evidenced by the lack of channels or any associated coastal plain deposits.

The O'Brien Spring delta was supplied with sediments from a west-northwest source, as indicated by the eastwards paleocurrents west of the study area. However, the delta system was re-aligned by southerly-oriented tidal currents within a lowstand-narrowed Western Interior Seaway, probably in a similar way (though on a smaller scale) as occurs in the modern Klang delta of Malaysia. It is also likely that the delta system prograded this far into the seaway during falling sea-level, argued from its regressive character, flattish shoreline trajectory, and non-association with any tail of coastal-plain deposits (it would seem to be subaqueously detached from the coeval shoreline). Modest sea level falls (a few 10s of m) on a very flat alluvial-shelf landscape forced deltaic shorelines into the seaway by up to 150 km beyond the western highstand shorelines, thus creating a narrower seaway and likely enhancement of southerly-oriented tidal currents. This scenario of decreased basin water depth would also have built more irregular shorelines and enhanced basin-floor topography, both of which would have amplified the tidal processes. The falling-stage to lowstand O'Brien Spring clinoforms were possibly the outer segment of a double, larger-scale clinoform, whose inner segment was the slightly older highstand, wave-dominated shorelines farther west.

AAPG Search and Discovery Article #90163©2013AAPG 2013 Annual Convention and Exhibition, Pittsburgh, Pennsylvania, May 19-22, 2013