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

Abstract

Shoreline clinoforms are stratal surfaces that represent the depositional gradient, at a given moment, of a subaerial to subaqueous marine profile and their associated facies belt for each segment. The reconstruction and characterization of clinoforms provide an estimate of paleobathymetry, one of the most difficult attributes to extract from sedimentary facies.The fine-grained sandstones to mudstones of the Pilmatué Member (late Valanginian-early Hauterivian) represent a siliciclastic dominated, shoreface-to-basin system, developed in a ramp 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. The objective of this contribution is to present a methodology for reconstructing of shoreface-to-basin clinoforms based on an outcrop case study of parasequence scale and to discuss the paleobathymetric implications.In order to do this, the width of the facies belts that conform the subaqueous part of the clinoform was firstly considered: Upper shoreface (Us), Lower shoreface (Ls), Offshore transition (Ot), Offshore (Of), Basin (Ba). They were estimated from a 20 km-long correlation panel 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 Pilmatué was calculated from a statistical approach using metadata analysis. The dataset used for this analysis was built including 7 cases of modern systems similar to the Pilmatué setting and one from fossil record resulting in a set of data with two variables, facies belt width and its 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 within average plus and minus standard deviation limits. Then, the gradient was determined.The most representative reconstructed Pilmatué clinoform in the northern area have the following width and gradient: US of 0.74 km and 0.70°; LS of 2 km and 0.30°; Ot of 3 km and 0.04°; Of of 8.5 km and 0.02°; BA of 7 km and 0.01°. the cumulative water depth at the end of each segment can 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 to the significant decrease in depositional gradient. Therefore, together the US and LS segments are interpreted to represent the foreset component of the shoreline clinoform, and the 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 considerably lower than previous suggestions. The results of this detailed reconstructed clinoform highlights the need of independent data in order to provide confident paleobathymetric estimations for shoreface-to-basin depositional systems, particularly those developed in intracontinental seaways.