UPPER CRETACEOUS GEOLOGY OF FRESNO, MADERA AND MERCED COUNTIES

Abstract

Abstract The stratigraphy of the Cretaceous section of the Northern San Joaquin basin is composed of deep and shallow marine, detrital clastics, lagoonal, and delta facies. The sands are massive in the delta front with thin shales and small amounts of lignite. Westward in the deeper neritic zone an intermediate sand/shale sequence develops from longshore transport and basinward dispersal of the clastic sediments. In the deeper bathyal and abyssal environments the prodelta shales merge with deep water turbidite sand. Four depositional systems have been recognized in the study area: 1) Eastern fluvial system, 2) An eastern deltaic system, 3) Central shelf-slope system, and 4) Submarine canyon and fan system. With the uplift of the ancestral Sierra Nevada Mountains, compressional forces caused reverse faulting of the subjacent metasediment series and late Cretaceous sediments. These tectonic forces lead to northwest-southeast trends of anticlinal type structures that are prevalent in Gill Ranch Gas, Moffat Ranch Gas, and Chowchilla Gas fields. These faulted anticlines, or paleo highs created the “shelf-slope” environment at the western edge of the productive trend. The Starkey sands are suggested to be delta front sands deposited along the edge of the shelf/slope system. The Lathrop, Winters, and Tracy sands are considered to be deep marine turbidite deposits sourced from the cyclic western migrating Starkey delta. The Blewett sands of the Moreno Formation are submarine turbidite sands deposited in a regressive cycle of sedimentation in conjunction with the deposition of the Moreno shale member.