Deglacial and Postglacial Sedimentary Architecture in a Deeply Incised Palaeovalley: The Late Carboniferous (Pennsylvanian) Jejenes Formation, San Juan, Argentina

Mason Dykstra, Institute for Crustal Studies, University of California, Santa Barbara, CA 93106, phone: 8058938435, dykstra@crustal.ucsb.edu, Benjamin Kneller, Geology and Petroleum Geology, University of Aberdeen, Aberdeen, AB24 3UE, United Kingdom, and Juan Pablo Milana, CEAZA, Universidad de La Serena, P.O. Box 599, La Serena, Chile.

Quebrada de las Lajas, San Juan, Argentina, preserves a mid-Carboniferous deglacial succession in a paleofjord. The sedimentary succession can be divided into four distinct stages. The first is characterized proximally by Gilbert-type deltas and distally by sheet-like sandbodies, varved siltstones and channel sandstone and conglomerate bodies. Dropstones are present, but their abundance decreases upward. Depositional environments varied from ice-contact deltas, subaqueous outwash fans, and related deep-water environments. Mass-transport deposits are abundant near the top of Stage 1, up to 50 m thick, and hundreds of meters wide and long. They exhibit significant surface topography (> 20 m), which was a major control on subsequent sediment pathways. Stage 2 records a glacioeustatic marine transgression, and a reduction of clastic supply. It is characterized by dark, organic-rich marine shales that could form significant local source rock (type III). These are interbedded with rare, thin turbidite sandstones and conglomerates. Stage 3 records progressive infilling of the accommodation space created by glacial overdeepening and glacioeustatic transgression. It is characterized by thick-bedded (0.5-10 m) sheet-like turbidite sandstones and associated shales. Stage 4 is interpreted as a fan-delta, and is characterized by coarse turbiditic sandstones and conglomerates interbedded with abundant, small-scale mass-transport deposits. This stage represents a major rejuvenation of sediment influx into the paleofjord. The form of the paleofjord floor created a depositional/compactional syncline, with pinch-outs toward the margins, and dramatic thickening toward its centre, forming effective structural/stratigraphic traps.

Permo-Carboniferous (Classic Gondwana) Basins and Reservoirs
2006 AAPG International Conference and Exhibition, (November 5-8, 2006) Technical Program