Driving processes and recognition criteria for complex subaerial unconformities in a non‐marine succession: Implications for basin margin shaping during forced regressions

VEIGA, GONZALO D.; VEIGA, GONZALO D.; SCHWARZ, ERNESTO; SCHWARZ, ERNESTO; OLIVO, MARIANA S.; OLIVO, MARIANA S.

SEDIMENTOLOGY

WILEY-BLACKWELL PUBLISHING, INC

Lugar: Londres; Año: 2020 vol. 67 p. 2777 - 2817

0037-0746

A regional sedimentological and sequence stratigraphic analysis is presented inthis paper to provide new insights into the morphological complexity and sedimentarymechanisms of unconformities giving shape to basin margins. Althoughthe development of unconformities entails significant sediment delivery to beconsidered when analysing ?source to sink? systems, as well as large palaeogeographicalchanges related to basin reconfiguration episodes, the complexity thatunconformities can reveal towards basin margins has been scarcely explored. Asedimentological and architectural analysis combining outcrop and subsurfacedatasets shows five palaeoenvironmental evolutionary stages and four key stratigraphicsurfaces in a constrained ca 2 Ma Valanginian interval on the southernmargin of the Neuquen back-arc basin (Argentina). A complex-type subaerialunconformity was identified, composed in turn of two subaerial unconformitiesexhibiting both single and composite motifs, different morphologies and faciesshifts representative of large palaeogeographical changes. In the studied stratigraphicinterval, two erosional stages occur linked to combinations of exclusivelynon-marine-driven processes involved in unconformities developing. Thetwo subaerial unconformities differ in nature and distribution, representing anovel case of complex unconformities and stratigraphic architectures in nonmarinelowstand wedges. The Valanginian complex subaerial unconformityentails a high diachroneity along strike and depositional dip, implying that thehiatus created in landward settings occurred during relative sea-level fall andrise stages during a period longer than in basinward areas. Disagreeing with classicalsequential models, two third-order sequence components of a complexlowstand wedge are preserved in proximal settings. Subsidence-controlledaccommodation and interplay between second-order and third-order cycleswere combined, increasing the prospects of sediment storage and preservationpotential of composite sequences towards landward areas. This work improvescurrent comprehension about complex subaerial unconformities formation andrelated lowstand architectures in proximal settings, providing criteria to understandand revaluate lowstand wedge geometries, particularly for more complexexamples, such as the case reported in this contribution.