A high-resolution record of deepwater processes in a confined paleofjord, Quebrada de las Lajas, Argentina

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

AAPG MEMOIR

AAPG

Lugar: Tulsa; Año: 2007 vol. 56 p. 42 - 44

0271-8529

Fjords can become so over-deepened below sea level during protracted glacial cycles that they fill with hundreds of meters of seawater when glacio-eustatic rise occurs during and following deglaciation. Fjords, therefore, can host true deep-water environments, in what are commonly laterally confined but longitudinally extensive settings. Ancient outcrops of paleofjord sediments offer three-dimensional views of the evolution of these deepwater, confined sedimentary environments, where the forcing factors controlling sediment supply are both climatic (deglaciation, eustasy) and tectonic (isostatic rebound) in origin. Quebrada de las Lajas, near San Juan, western Argentina, preserves a Late Carboniferous (Pennsylvanian) glacial to postglacial succession that was deposited in an over-deepened paleofjord. The sedimentary succession exposed in the paleofjord is divided into four evolutionary stages: Stage I was an ice-contact delta and proglacial lake; Stage II was a relatively quiet, deepwater marine environment punctuated by turbidity currents; Stage III was a aggradational confined sheet system; and Stage IV was the subaqueous portion of a progradational, coarsening and shoaling-upward fan-delta. The entire sedimentary succession comprises approximately 350 m (1150 ft) remaining exposed thickness. Each of the four evolutionary Stages has distinct architectural characteristics associated with its depositional environments. Stage I is characterized by predominantly lobeshaped, sheet-like conglomerates and sandstones associated with the ice-contact delta and a subaqueous fan. Stage I also preserves a number of small turbidite channel bodies and a small-scale, highly aggradational channel-levee system with a conglomeratic channel-axis and thin-bedded sandstone and siltstone levees. Stage II is characterized by thin-beds of shale and siltstone punctuated by medium beds of sandstone and conglomerate. All of these beds have sheet-like, unconfined geometries. There are also a number of small, locally confined channel bodies within Stage II that are characterized by turbidite sandstone fills and often are located on top of Stage I mass-transport deposits. Stage III is characterized by ubiquitous, thick, sheet-like turbidite sandstone beds with a very blocky character, which thin toward the margins of the paleofjord. Stage IV is characterized by more localized (lobe-shaped?) sheet-like sandstone and conglomerate bodies. Mass-transport deposits are common throughout the paleofjord fill. They include deformed beds, chaotic and mixed gravels, sands, and fine-grained successions, and slumps and slides of various scales, including slump bodies over 500 m long and wide, and over 50 m thick. Mass-transport deposits are present in all stages, but are most abundant in Stages I and IV.