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

Abstract

Fjords can become so overdeepened below sea level during protracted glaciations 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, which are commonly laterally confined but longitudinally extensive. Outcrops of ancient paleofjord sediments offer three-dimensional views of the evolution of these deep-water, confined sedimentary environments, where the factors controlling sediment supply are both climatic (deglaciation and eustasy) and tectonic (isostatic rebound) in origin. Quebrada de las Lajas, near San Juan, western Argentina, preserves a Pennsylvanian glacial to postglacial succession that was deposited in an overdeepened 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, deep-water marine environment punctuated by turbidity currents, Stage III was an aggradational conf ined sheet system, and Stage IV was the subaqueous part of a progradational, coarsening- and shoaling-upward fan delta. The entire sedimentary succession comprises approximately 350 m (1150 ft) of remaining exposed thickness. Each of the four evolutionary stages has distinct architectural characteristics associated with its depositional environments. Stage I is characterized by predominantly lobe-shaped, sheetlike conglomerates and sandstones associated with the ice-contact delta and a subaqueous fan. Stage I also preserves several 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 sheetlike, unconfined geometries. Several small, locally confined channel bodies within Stage II are characterized by turbidite sandstone fills and commonly are located on top of Stage I mass-transport deposits. Stage III is characterized by ubiquitous, thick, sheetlike 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?), sheetlike 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 >500 m (>1640 ft) long and wide, and more than 50 m (164 ft) thick. Mass-transport deposits are present in all stages, but are most abundant in Stages I and IV.