Deglacial and Postglacial Sedimentary Architecture in a deeply incised paleovalley; the late Carboniferous (Pennsylvanian) Jejenes Fm., San Juan, Argentina.

DYKSTRA, M., KNELLER, B., AND MILANA, J.P.

Perth, Australia

Congreso; AAPG International Conference and Exhibition, (November 5-8, 2006), Perth, Australia; 2006

AAPG

Quebrada de las Lajas, San Juan, Argentina, preserves a mid-Carboniferous deglacial succession in a highly confined paleofjord. The sedimentary succession can be divided into four distinct Stages. The first Stage is characterized proximally by Gilbert-type deltas and distally by sheet-sandbodies, varves, and interbedded 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 (MTDs) are abundant near the top of Stage one. These MTDs vary up to 50 m thick, and hundreds of meters wide and long. They exhibit significant surface topography (> 10 m), which was a major control on subsequent sediment pathways in the paleofjord. Stage two records a glacioeustatic marine transgression, and a shut-off or slow-down of the clastic conduit in the paleofjord. This Stage is characterized by dark, organic-rich marine shales quite unlike the usual green shales present in the paleofjord. These could be a significant local source rock (type III). The dark shales are interbedded with rare, thin turbidite sandstones and conglomerates. The third Stage records progressive infilling of the accommodation space created in the paleofjord due to glacial overdeepening and the glacioeustatic marine transgression. Stage three is characterized by thick-bedded (0.5-10 m) sheet-like turbidite sandstones and associated shales. The fourth and final Stage is interpreted as a fan-delta, and is characterized by coarse turbiditic sandstones and conglomerates interbedded with abundant, small-scale MTDs. This Stage represents a major rejuvenation of sediment influx into the paleofjord. The shape of the paleofjord created a depositional/compactional syncline, where beds pinch-out toward the margins of the paleofjord, and thicken dramatically toward its centre, which forms very effective structural/stratigraphic traps. Additionally the large structures in the MTDs form traps with three-dimensional closures over large portions of the paleofjord.