GRABOWSKI, GEORGE J., JR, and JAMES H. ANDERSON, Exxon Exploration Company, Houston, TX

ABSTRACT: Sequence Stratigraphy of Carbonate-Ramp Sediments of the Upper Jurassic Smackover Formation, USA Gulf Coast Region

The Smackover Formation is a time-transgressive carbonate deposited in a ramp setting. It is composed of two sequences that are separated by a rapid basinward shift in shallow-marine sedimentation at the sequence boundary. Each sequence consists of backstepping, onlapping transgressive deposits, aggradational to progradational highstand parasequences, and aggradational lowstand parasequences. Sequence boundaries are characterized seismically by toplap and minor truncation in updip areas of rapid sedimentation, and by onlap of overlying sequences in downdip areas. Mid-sequence downlap surfaces are prominent above the transgressive deposits. Mounded facies occur in the early highstand of the lower Smackover, whereas shingled progradational packages are common during late highstands.

These geometric relationships are confirmed by detailed interpretations of facies and depositional environments. A gap in the normal facies progression from marine to nonmarine environments marks the sequence boundary in updip areas. Oolitic grainstones that formed active shoals are directly overlain bycoastal-plain clastics, without intervening lagoonal mudstones, oolitic beach grainstones and sabkha-salina evaporites that occur in downdip transitions. The sequence boundary is onlapped by oolitic grainstones or deltaic clastic rocks. Local basins rimmed by oolitic shoals contain deep-water sediments overlain by thick deposits of sabkha evaporite.

The basal Smackover consists of black, laminated, organic-rich mudstones which onlap the Norphlet Sandstone or older formations. These onlapping transgressive sediments were deposited in calm, restricted water, but locally they are interbedded with coral boundstones, stromatolitic mudstones and oolitic grainstones, suggesting deposition in shallow water. The transgressive deposits are overlain by several thick, widespread, aggradational parasequences of deeper water, bioturbated wackestones of the early highstand. Late highstand sediments consist of progradational parasequences of oolitic grainstones, behind which occur lagoonal and sabkha sediments.

AAPG Search and Discovery Article #90987©1993 AAPG Annual Convention, New Orleans, Louisiana, April 25-28, 1993.