Johanna J. Moutoux¹

(1) University of Wyoming, Laramie, WY

ABSTRACT: The Santonian Hosta-Dalton Sandstone, Hagan Basin, New Mexico: Outcrop Analog for a Shoreface Pinchout

Within the Hagan Basin of north central New Mexico, regressive Dalton and transgressive Hosta Sandstones converge as the basinward pinchout of a third-order northerly-prograding clastic wedge deposited in the Upper Cretaceous Western Interior Seaway. Detailed sequence stratigraphic analysis of seventeen sections measured on this approximately 12 km long pinchout sandstone reveals a complex record of high-order relative sea level oscillations expressed as three, possibly fifth-order, sequences. The basal sequence is bound at the base by a sequence boundary/tidal-ravinement surface and is composed of transgressive barrier-bar/lagoon deposits overlain by regressive prograding strandplain shoreface deposits, separated by a transgressive wave-ravinement surface. The two subsequent sequences are each composed of a thin transgressive sand and overlying prograding strandplain shoreface and are bounded by transgressive wave-ravinement surfaces. Flooding surfaces that cap regressive parasequences are observed as essentially horizontal in strike section. However, in dip section these same flooding surfaces illustrate the progradational nature of the shoreface deposits, exhibiting initially steep dips of .3° - .52° that distally shallow to .1°. The sandbodies within each prograding parasequence thin basinwards with concomitant thickening of the offshore to transitional finer-grained deposits above and below, creating a pinchout zone 6 - 12 km wide. An awareness of the landward and basinward thickening and thinning trends of sandstones and shales both along strike, and notably in dip section, is critical to characterize and predict the degree of reservoir compartmentalization. The Hosta-Dalton Sandstone provides a useful model for the behavior of higher-order shoreface progradation within a third-order stratigraphic pinchout.

AAPG Search and Discovery Article #90906©2001 AAPG Annual Convention, Denver, Colorado