--> Abstract: A Tide-Dominated Delta System In The Western Cretaceous Seaway: The Coniacian Tocito Sandstone, by G. W. Riley and D. Nummedal; #90928 (1999).

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RILEY, GREGORY W.1 and DAG NUMMEDAL2
1Amoco Production Company, Houston
2Unocal Corp., Sugar Land, TX

Abstract: A Tide-Dominated Delta System in the Western Cretaceous Seaway: The Coniacian Tocito Sandstone

Interpretation of coarse-grained, mudstone-encased, linear sandbodies in the Cretaceous Western Interior has remained controversial. This controversy continues in spite of decades of studies of these oil reservoirs. The principal dilemma is how to interpret high-energy coarse-grained sandstones seemingly deposited parallel to and many miles from the adjacent shoreline.

A comprehensive study incorporating detailed outcrop sections, subsurface well logs, and biostratigraphic data has revealed a complicated depositional history for the Tocito Sandstone. The critical element in this interpretation is the recognition of a lower, coarsening upward, progradational muddy sandstone capped by coarse-grained tidal channels and sand waves within the Tocito Sandstone. This lower Tocito interval fills structural lows (grabens) formed adjacent to uplifted horst blacks. On adjacent uplifted blocks the lower Tocito interval is absent by both onlap and erosional truncation. Overlying and erosionally truncating the lower Tocito is the upper Tocito. The upper Tocito overlies a horizontal erosion surface, contains tidal, cross-bedded sandstones and forms linear, elongate ridges.

The Tocito Sandstone represents a response to tectonic activity. Local uplift created near-shore bathymetric variations and amplification of tidal currents. Strong tidal currents resulted in progradation of the lower Tocito in a tide-dominated delta system. High-energy, coarse-grained tide-delta facies prograded many kilometers from the adjacent shoreline. Later, transgressive erosion removed these lower regressive deposits from structural highs. This sediment was then remolded into transgressive shelf sand ridges blanketing much of the Four Corners Platform.

AAPG Search and Discovery Article #90928©1999 AAPG Annual Convention, San Antonio, Texas