Fjords can become so overdeepened below sea level during protracted glaciations that they fill with hundreds of meters (>1000 ft) of seawater when glacioeustatic 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). Quebrada de las Lajas, near San Juan, western Argentina, preserves a 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, deep-water marine environment punctuated by turbidity currents, Stage III was an 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) of remaining exposed thickness.

Each of the four evolutionary stages has distinct architectural characteristics associated with its depositional environment. Stage I is chatacterized by predominandy lobe-shaped, sheetlike conglomerate and sandstone bodies 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