Variability of continental depositional systems during lowstand sedimentation: an example fron the Kimmeridgian of the Nauquén Basin, Argentina

SPALLETTI, L.A.; VEIGA, G.D.

LATIN AMERICAN JOURNAL OF SEDIMENTOLOGY AND BASIN ANALYSIS

Asociación Argentina de Sedimentología

Lugar: La Plata; Año: 2007 vol. 14 p. 85 - 104

1669-7316

Several second order lowstand wedges are recognized in the Jurassic-Early Cretaceoussedimentary record of the backarc Neuquén Basin (central-west Argentina). They are distinguishedby sharp based continental and marginal marine siliciclastic deposits encased in offshore shales.The Kimmeridgian lowstand wedge was developed slightly after the emergence of the Andeanmagmatic arc and the tectonic inversion of previous intrabasinal extensional structures. As a result,the Neuquén Basin was compartmentalized into three main depocentres characterised by widespreadcontinental sedimentation under arid to semiarid climatic conditions. A fluvial-dominated systemcharacterised by systematic downstream changes in architectural style is recognized in theNorthwestern Depocentre. A gravely and sandy bedload fluvial system was developed in the southernupstream sector, while ticker beds of finer-grained sediments formed in a distal ephemeral fluvialsystem prevail in the downstream part of the system. The overall fining upward stacking pattern ofthe sedimentary record in the Northwestern Depocentre accompanied by frequent development ofsoil horizons and darker deposits suggests a change towards higher accommodation and highwater table emplacement. In the Southwestern and Eastern Depocentres, the sedimentary successionsshow a conspicuous internal transition from fluvial ephemeral fluvial systems to aeolian systems.However, the lowstand deposits of the Eastern Depocentre are characterised by a larger arealdistribution and a thicker record of both the fluvial and the aeolian deposits. Marked changes inthickness and in the depositional style of the fluvial and aeolian facies associations within theSouthwestern and Eastern depocentres indicate that the sedimentary infill was controlled bysystematic variations in accommodation. Low accommodation conditions favoured a high degree oflateral migration of fluvial channels with substantial erosion of fine-grained deposits and thedevelopment of sinuous-crested aeolian dunes typically associated with wet interdune deposits.Under higher accommodation conditions the fluvial deposits show a retrogradational stacking withpreservation of thick packages of fine-grained sediments, while a large sand sea characterised byamalgamation of dune deposits was developed in the aeolian-dominated uppermost successions.The detailed analysis of the Kimmeridgian lowstand wedge of the Neuquén Basin illustrates howfacies and stratigraphic organisation responded to regional and temporal changes in basinconfiguration, accommodation, sediment supply and water table position. The Kimmeridgianlowstand deposits are geographically distributed as the subsequent transgressive deposits andreveal no major basinward shift during the early stages of sequence stacking. However, they showa much more complicated facies distribution. Consequently, the lowstand wedge deposits betterreflect the complex interplay of episodic local tectonism, siliciclastic source area variation andclimatic change.