Cari L Johnson¹, Stephan A. Graham¹, Marc A. Hendrix²
(1) Stanford University, Stanford, CA
(2) University of Montana

Abstract: Sedimentary characteristics and stratigraphic architecture of a Mesozoic lacustrine deltaic sequence, southern Mongolia

In southeastern Mongolia, deep-water lacustrine deposits and associated perilacustrine deltaic and fluvial sequences represent infilling of the East Gobi basin during Late Mesozoic intracontinental rifting. Along the northern margin of the basin at Har Hotol, erosion at the core of a gently plunging anticline forms a natural ‘amphitheatre’ of dramatic cliff exposures. Outcropping continuously for several kilometers, a 30 m-thick section of fine-grained, quartzose sandstone, siltstone, and shale records a prograding lacustrine delta. The section includes upward thickening and coarsening sandstone beds characterized by pervasive soft sediment deformation and cm-scale asymmetric and climbing ripples, interpreted as pro-delta deposits. The upper half of the section is dominated by broad (3-10 m wide by 1-3 m tall) channels that migrated laterally across the delta plain, as indicated by complex stacking and truncational relationships. Paleocurrents are southwest-directed, and along the western edge of the amphitheatre in the inferred offshore direction, outcrops are noticeably finer-grained, with thinly-bedded turbidites. Similar stacked prodelta patterns are characteristic of much of the Lower Cretaceous synrift sequence, and these units represent analogs for subsurface reservoirs in the nearby petroliferous Unegt and Zuunbayan sub-basins. Photo-panorama mapping and 12 detailed, GPS-controlled, measured sections define the main architectural elements of the deltaic sequence and document the complex sedimentary characteristics and geometric heterogeneities of this depositional system.

AAPG Search and Discovery Article #90914©2000 AAPG Annual Convention, New Orleans, Louisiana