Sequence Boundary Modification by Submarine Erosion, Upper Cretaceous, San Juan Basin

RILEY, GREGORY W., and DAG NUMMEDAL, Louisiana State University, Baton Rouge, LA

Unconformities within shallow-marine depositional systems are commonly interpreted as exclusively a result of erosion during falling relative sea level. However, significant submarine erosion also occurs during transgressions and is a complicating factor in the interpretation of these unconformities. A major unconformity present in the latest Turonian and Coniacian regressive-transgressive cycle in the northwestern New Mexico, separates the Gallup Sandstone (HST) from the overlying Tocito Sandstone (TST and rarely LST). The basinward portion of the unconformity contains no evidence of subaerial exposure and is contained entirely within marine strata. Also, the associated lacuna decreases paleolandward. Above the unconformity, although there is addition of coarser sand grains, overlyin strata contain concretions, glauconite and phosphate nodules, and are more heavily burrowed than underlying strata. These relationships suggest that the preserved unconformity reflects shoreline transgression and a decrease in the rate of allochthonous sedimentation. The stratigraphic relationships are interpreted in the following manner. HST of the Gallup Sandstone prograded from southwest to northeast forming a series of wave-dominated shorelines. A subsequent fall in relative sea level resulted in a forced regression and the formation of coarse-grained delta "plumes" (some Tocito sandbodies) and inferred lowstand deltas. Sea-level rise, following lowstand, resulted in regional transgression. Erosion of previously deposited strata was accomplished by both shoreface erosion (ravinement and inner shelf erosion during the initial stages of this transgression. Erosion of the shelf was probably locally enhanced by the presence of promontories associated with lowstand deltas and uplift of isolated structural blocks. As transgression continued, inner-shelf sand ridges and sand wave fields (Tocito Sandstone) formed from previously deposited lowstand sandbodies.

AAPG Search and Discovery Article #91004 © 1991 AAPG Annual Convention Dallas, Texas, April 7-10, 1991 (2009)