Detailed Stratigraphy of Slope Deposits Associated with a Large Scale Mass Transport Complex – Chacay-Melehue, Neuquen Basin, Argentina

Oliveira, Carlos M.¹; Hodgson, David M.; Flint, Stephen; and Magalhaes, José Luis C.
¹[email protected]

During the Upper Triassic – Lower Jurassic, the Neuquén Basin, Argentina, underwent back-arc rifting, which led to the formation of distinct depocenters with the deposition of continental, volcaniclastics and marine turbiditic sediments. The objective of this study was to investigate the stratigraphic distribution and the possible local tectonic control on sediment deposition and deformation in the post-rift Los Molles Formation, Lower Jurassic, at the Cordillera del Viento, Chacay-Melehue.

A 60 m thick succession, exposed along a 7km dip section comprises four units that show a general coarsening and thickening upwards and are bound deep water shales. Unit 1 is 10 to 20m in thickness and is composed by chaotic deposits in the base followed by 3 coarsening and thickening upwards sedimentary packages. Unit 2 is 15 to 20m thick and comprises marine shales and thinly bedded sandstones (0.05-0.1m thick). Unit 3 comprehends a 10 to 20m sedimentary section, composed by thin-bedded sandstones at the base grading to coarser and thicker sandstones to the top. Frequently, Unit 3 is cut by erosive surfaces from the base of Unit 4, which comprises 15 to 20m of coarse sandstones and conglomerates.

The chaotic deposit is interpreted as a debrite, which is overlain by coarse turbiditic sandstones and conglomerates that infill depressions on the top of the debrite, without indication of erosion. The following sandstones packages show a down-dip thinning from west to east. The instability along the slope is evidenced by slumping and fluid escape structures, which are associated with detachment surfaces at the base of each unit. It is interpreted that the deformation was controlled by the downward movement of the sedimentary pile along the shaly slope. The possible triggers for the deformation can be associated with the Andean seismicity, the sedimentary load and the high angle of the slope gradient. Tectonic features are post-depositional, and are represented by normal faults along the North to South direction and by the thrust faults with Northwest-Southeast direction, which are related to the inversion of normal faults.

AAPG Search and Discovery Article #90166©2013 AAPG International Conference & Exhibition, Cartagena, Colombia, 8-11 September 2013