Abstract: Deltaic and Interdeltaic Sedimentation, La Ventana Tongue of Cliff House Sandstone (Upper Cretaceous Mesaverde Group), Southeastern San Juan Basin, New Mexico

G. W. Mannhard

The Late Cretaceous depositional history of the La Ventana Tongue of the Cliff House Sandstone and adjacent units is described from detailed outcrop studies. The La Ventana Tongue (up to 960 ft thick) is underlain by, and toward the south intertongued with, the nonmarine Menefee Formation, and is overlain by, and toward the north intertongued with, the marine Lewis Shale. The lower part of the La Ventana Tongue was deposited in deltaic-distributary channel and destructional-beach environments. The upper La Ventana records cyclic deposition in which sequences of neritic shale (transgressive), shoreface and foreshore beach sandstones are repeated vertically. The Menefee Formation consists mainly of deltaic coastal-plain deposits including channel sandstones, coals of swamp rigin, and organic-rich shales of marsh and bay origin.

Distributary-channel sandstones are bounded by marsh deposits and characterized by lenticular shape, scour and/or loading structures, penecontemporaneous faulting at their margins, internal-deformation structures, and down-to-basin slumping. Shoreface deposits consist of well-sorted, very fine- to fine-grained, widespread, uniformly thick sandstones, which are interbedded with marine shales at their base and characterized by well-defined coarsening-upward grain-size trends, trough cross-stratification, and abundant Ophiomorpha. Locally shoreface sandstones grade upward into foreshore beach sandstones characterized by subparallel, gently dipping stratification.

Regional stratigraphic relations of marine and nonmarine facies indicate a source area in the south or southwest. The shoreline trend was northwest as indicated by the orientation of symmetrical straight-crested ripple marks in shoreface, and subshoreface sandstones. The dominant direction of longshore transport was to the northwest as recorded by trough cross-stratification in shoreface sandstones.

AAPG Search and Discovery Article #90979©1975 AAPG – SEPM Rocky Mountain Sections Annual Meeting, Albuquerque, New Mexico