EFFECTS OF SEA LEVEL CHANGE ON DEPOSITION IN THE SEVIER FORELAND BASIN

Evidence for the occurrence of multiple episodes of sea level fall is abundant and well documented in strata of the Sevier foreland basin (FLB) and the contemporaneous Cretaceous Western Interior Seaway (KWIS). Facies evidence from marine strata in the Sevier foreland basin supports an interpretation of sea level fall with a frequency of up to 0.5 Ma in the Campanian. On the eastern margin of the KWIS, sea level fall documented in the Turonian resulted in significant narrowing of the seaway, sometimes migrating the eastern margin of the seaway to within the position of the highstand axis of the seaway. These events greatly reduced the width of the entire seaway and extent of marine incursion in the adjacent Sevier foreland basin. Facies interpretations and strontium isotopic data support westward shoreline translation of the eastern margin of the seaway by 700 to 1100 km during lowstand events. This lowstand migration could have resulted from a sea level fall of only 30-60 meters. Lowstand events of significant magnitude resulted in a narrowing of the KWIS, the exposure of the forebulge of the Sevier FLB, and enhanced axial parallel drainage in the FLB. During periods of significant lowstand events, the marine incursion into the Sevier FLB is proposed to be decoupled from marine waters of the KWIS. Increased erosion resulting from increased fluvial gradients during episodes of sea level fall provided more sediment for the lowstand system tracts. Although incised valley deposits are plentiful in the proximal foredeep of the Sevier FLB, lowstand shorelines are difficult to identify anywhere in the FLB. We propose that the retreat of marine waters from the foredeep augmented sediment supply for lowstand events in the Gulf of Mexico. This might have been a main source of sediment for Cretaceous lowstand deposits in the western portion of the Gulf of Mexico.