Clinoform reconstruction for the Pilmatué Member (Neuquen Basin): implications for paleobathymetry of an interior sea

ZUAZO, J.J.; SCHWARZ, E.; VEIGA, G.D.

Roma

Congreso; 35th IAS Meeting of Sedimentology; 2019

International Association of Sedimentologists

Shoreline clinoforms are stratal surfaces that represent the depositional gradient, at a givenmoment, of a subaerial to subaqueous marine profile and their associated facies belt foreach segment. The reconstruction and characterization of clinoforms provide an estimate ofpaleobathymetry, one of the most difficult attributes to extract from sedimentary facies.The fine-grained sandstones to mudstones of the Pilmatué Member (late Valanginian-earlyHauterivian) represent a siliciclastic dominated, shoreface-to-basin system, developed in aramp type setting (i.e., without shelf break) of a back-arc interior sea, the Neuquén Basin(Argentina). A water depth of 50-200 m has been commonly assumed for the distal (basinal)part of this system, although no hard data has been provided to support this estimation. Theobjective of this contribution is to present a methodology for reconstructing of shoreface-tobasinclinoforms based on an outcrop case study of parasequence scale and to discuss thepaleobathymetric implications.In order to do this, the width of the facies belts that conform the subaqueous part of theclinoform was firstly considered: Upper shoreface (Us), Lower shoreface (Ls), Offshoretransition (Ot), Offshore (Of), Basin (Ba). They were estimated from a 20 km-long correlationpanel perpendicular to shoreline orientation and measured at the top of a parasequence(below flooding surface). Then, the depositional gradient for each facies belt of the Pilmatuéwas calculated from a statistical approach using metadata analysis. The dataset used forthis analysis was built including 7 cases of modern systems similar to the Pilmatué settingand one from fossil record resulting in a set of data with two variables, facies belt width andits depositional gradient. Also, in the only case that facies belt of the Pilmatué was absent(Us) the facies belt was determined considering the average value of available values withinaverage plus and minus standard deviation limits. Then, the gradient was determined.The most representative reconstructed Pilmatué clinoform have the following width andgradient: US of 0.74 km and 0.70°; LS of 2 km and 0.30°; Ot of 3 km and 0.04°; Of of 8.5km and 0.02°; BA of 7 km and 0.01°. the cumulative water depth at the end of each segmentcan be therefore calculated, being -9 m, -19 m, -21 m, -25 m, -26 m respectively. Together,Us and Ls segments (2.75 km) represent 73% of the total estimated water depth (-19 m),whereas across the remaining 18.5 km, the system only deepens an additional 7 m due tothe significant decrease in depositional gradient. Therefore, together the US and LSsegments are interpreted to represent the foreset component of the shoreline clinoform, andthe sector of the Ot segment would represent the transition to the bottomset. Additionally,the -25 m estimated water depth for the distal part of the system (Of-Ba limit) is considerablylower than previous suggestions.The results of this detailed reconstructed clinoform highlights the need of independent datain order to provide confident paleobathymetric estimations for shoreface-to-basindepositional systems, particularly those developed in intracontinental seaways.