Clinoform Geometries, Facies Distributions, and Exploration Significance of a Campanian Spit, Billings, Montana
HANSON, MARK S., Shell Western E&P, Inc., Houston, TX
A Campanian spit comprised of clinoform-bound sandstone wedges demarcates the base of the Eagle Sandstone along the "rimrocks" surrounding Billings, Montana. The spit is connected to the Eagle strandline via a seismically defined northwest-southeast-trending basement fault which cores the Crooked Creek anticline. This recurrently active fault cut perpendicular to the Eagle strandline, localizing a fluviodeltaic headland which delivered an enormous volume of sand to the pendant spit.
Detailed facies analysis of outcrop and subsurface data documents a strandline-parallel, southwesterly offlap of the sandstone wedges on the shallow shelf. Facies progressions within and across accretionary wedges define relative sea level movements during the genesis of the spit. Longshore and eddy currents, combined with a slow eustatic rise, forced a southwesterly migration of fine- to medium-grained sand. Spit progradation slowed and was ultimately terminated as the rate of eustatic rise overtook the rate of sediment supply.
Spits resulting from similar/fault intersections are excellent exploration targets. The Billings spit spans 180 mi2 and reaches thicknesses exceeding 100 ft. A favorable reservoir/trap configuration results from the spit's lensoidal and potentially shale-encased geometry. Exploration for these spits must concentrate seaward and downcurrent of near-perpendicular fault/strand intersections.
AAPG Search and Discovery Article #91010©1991 AAPG Rocky Mountain Section Meeting, Billings, Montana, July 28-31, 1991 (2009)