Hyperpycnal delivery of sand to the continental shelf: Insights from the Jurassic Lajas Formation, Neuquén Basin, Argentina

Abstract

Hyperpycnal currents are river‐derived turbidity currents capable of transporting significant volumes of sediment from the shoreline onto the shelf and potentially further to deep ocean basins. However, their capacity to deposit sand bodies on the continental shelf is poorly understood. Shelf hyperpycnites remain an overlooked depositional element in source to sink systems, primarily due to their limited recognition in the rock record. Recent discoveries of modern shelf hyperpycnites, and previous work describing hyperpycnites deposited in slope or deep‐water settings, provide a valuable framework for understanding and recognizing shelf hyperpycnites in the rock record. This article describes well‐sorted lobate sand bodies on the continental shelf of the Neuquén Basin, Argentina, interpreted to have been deposited by hyperpycnal currents. These hyperpycnites of the Jurassic Lajas Formation are characterized by well‐sorted, medium‐grained, parallel‐laminated sandstones with hundreds of metre extensive, decimetre thick beds encased by organic‐rich, thinly laminated sandstone and siltstone. These deposits represent slightly obliquely‐migrating sand lobes fed by small rivers and deposited on the continental shelf. Hyperpycnites of the Lajas Formation highlight several unique characteristics of hyperpycnal deposits, including their distinctively thick horizontal laminae attributed to pulsing of the hyperpycnal currents, the extraction of coarse gravel due to low flow competence, and the extraction of mud due to lofting of light interstitial fluid. Recognition of shelf hyperpycnites in the Lajas Formation of the Neuquén Basin allows for a broader understanding of shelf processes and adds to the developing facies models of hyperpycnites. Recognizing and understanding the geometry and internal architecture of shelf hyperpycnites will improve current understanding of sediment transfer from rivers to deeper water, will improve palaeoenvironmental interpretations of sediment gravity‐flow deposits, and has implications for modelling potentially high‐quality hydrocarbon reservoirs.

The hyperpycnite problem

“…During the past four decades, there has been an accelerated effort to understand these density plumes through (1) observational and/or interpretational (Arnau et al 2004;Bhattacharya and MacEachern 2009;Collins et al 2017;Gihm and Hwang 2016;Johnson et al 2001;Lewis et al 2018;Luo et al 2017;Milliman et al 2007;Mulder et al 2003;Mutti et al 1996;Ogston et al 2000;Pan et al 2017;Petter and Steel 2006;Pierce 2012;Puig et al 2014;Schillereff et al 2014;Shanmugam 2016aShanmugam , 2018aShanmugam , 2018bShanmugam , 2018cSoyinka and Slatt 2008;Steel et al 2016Steel et al , 2018Sun et al 2016;Talling 2014;Warrick et al 2013;Schieber 2014, 2017;Wright et al 1986Wright et al , 1988Yang et al 2017a;Zavala and Arcuri 2016;Zavala and Pan 2018;Zavala et al 2006; among others), (2) experimental (Kostic and Parker 2003;Kostic et al 2002;Lamb and Mohrig 2009;Lamb et al 2010;Parsons et al 2001), and (3) numerical (Chen et al 2013;…”

Facies and architectural variability of sub‐seismic slope‐channel fills in prograding clinoforms, Mid‐Jurassic Neuquén Basin, Argentina

“…An intermittent connection to the Pacific Ocean introduced a mesotidal range to the Neuquén back‐arc basin (Dean, ) that has left extensive tidal deposits on the shelf. These cross‐shelf tidal currents (Rossi et al, ) and hyperpycnal flows (Steel, Simms, Steel, & Olariu, ) contributed to sediment delivery into the Bey Malec slope channels, especially aiding the delivery of fine sediments during relative sea level highstands, albeit modest greenhouse highstands. Variations of palaeocurrent direction suggest that there were local changes of sediment source potentially associated with delta lobe switching and channel feeder avulsion, as well as autogenic evolution of deepwater slope channels (e.g.…”

River‐dominated and tide‐influenced shelf‐edge delta systems: Coarse‐grained deltas straddling the Early–Middle Jurassic shelf–slope break and transforming downslope, Lajas–Los Molles formations, Neuquén Basin, Argentina

“…These sandstone bodies have been interpreted by Steel et al . () as small hyperpycnal shelf fans built in front of small rivers that did not reach the shelf break. Note that lobes seem to stack basinward, likely indicating north‐east progradation (Fig.…”