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ABSTRACT: Longshore-Drift Dispersed, Storm-Generated Cross-Stratified Sandstone from Some Cretaceous Shallow Marine Strata, Rocky Mountain Region

Edmund R. Gustason

Most Cretaceous shallow marine strata of the Rocky Mountain region (Muddy, Dakota, Frontier, Teckla, Fox Hills formations and numerous tongues of the Mesa Verde Group) are characterized by asymmetrical,

upward-coarsening and upward-thickening sequences. The strata typically contain similar lithofacies (i.e., normally graded planar parallel laminated claystone, siltstone, and sandstone; hummocky cross-stratified sandstone; symmetrical and asymmetrical ripple cross-lamination; and trough and planar tublar cross-stratified sandstone) and display an upward increase in the thickness and frequency of sharp-based sandstone beds that grade into amalgamated cross-stratified sandstone. Most workers agree that sharp-based sandstone beds and hummocky cross-stratified sandstone are storm generated. However, the origin of trough and planar tabular cross-stratified sandstone is controversial. Most workers interpret these sedimentary structures as deposited from either storm-generated traction curre ts or combined flow currents on the shelf, tide-generated traction currents, or tidally dispersed, storm-generated suspension clouds.

Detailed analysis of three-dimensional outcrops has revealed several significant features of these sedimentary structures that indicate they may have been deposited by longshore drift dispersed, storm-generated suspension clouds. Sets of trough and planar cross-stratified sandstone form medium-scale discontinuous, irregularly shaped sand bodies, bound by erosional surfaces and composed of unidirectional dip-oriented cross strata. Individual cross stratum commonly have a sigmoidal shape, are bound by either reactivation surfaces or mudstone drapes, and contain normally graded concordant laminae. Top-set laminae are truncated by the upper set boundary, whereas bottom-set laminae become asymptotic to the lower set boundary and commonly are reworked and overlain by wave generated, ripple ross-lamination or mudstone drapes. Reactivation surfaces commonly truncate bioturbation. These trough and planar tabular cross-stratified sandstones lack inverse tangential and inverse perpendicular graded foreset laminae characteristic of traction current generated megaripples or sand waves. They also lack bundled sequences of coupled mud drapes which are characteristic of tidally generated megaripples or sand waves. Their close association with hummocky cross-stratified sandstone suggests that they record episodic erosion and deposition from suspension within storm wave base under the influence of a unidirectional current, probably longshore drift in the upper shoreface zone. Set boundaries are thought to record major storms which rework the entire upper shoreface zone.

AAPG Search and Discovery Article #91003©1990 AAPG Annual Convention, San Francisco, California, June 3-6, 1990