Distinguishing Channel Related Thin-Bedded Turbidite Facies – Outcrop Example From the Rosario Formation, Mexico
Thin-bedded turbidites deposited by overbank flow from submarine channels often contain significant volumes of laterally continuous sand that can form commercially important hydrocarbon reservoirs. Thin-bedded turbidites can be deposited in environments including levees and terraces, which are distinguished on the basis of their external and internal architecture. Levees can confine both channel thalwegs (internal levees) and channel belts (external levees) and decrease in height away from the channel. Terraces are elevated flat-lying features that lie above the active thalweg within a broad channel belt. Despite the ubiquity of terraces and levees in modern submarine channel systems the recognition of these environments in outcrop and in the subsurface is challenging. In this study of the Upper Cretaceous Rosario Formation (Baja California, Mexico), multiple logged sections that comprise a lateral transect of thin-bedded turbidites ranging from the channel belt to the levee were analyzed. Sedimentary logging reveals systematic trends in sand bed thicknesses, net to gross, palaeocurrents, sedimentary structures and ichnology that have been used to establish a framework for distinguishing thin-bedded turbidite facies of levees and terraces. Thin-bedded turbidites of channel belt terraces have a larger standard deviation in sand bed thicknesses than external levees. This is interpreted to be a consequence of the filtering effect of the height of the external levees, which limits the size range of turbidity currents that can overspill the confining levee. Terraces are topographically lower, and experience a wider range of sizes of turbidity currents. Sandstone bed thickness within external levees decreases away from the channel while bed thickness distribution is more regular across terraces. Terrace environments of the channel belt are characterized by high bioturbation intensities, and contain distinctive trace fossil assemblages, often dominated by the echinoid trace fossil Scolicia. These assemblages contrast with the lower bioturbation intensities that are recorded from external levee environments where Scolicia is absent. This analysis provides a number of criteria that help to characterize the thin-bedded turbidite facies of terraces and levees. These may help to improve recognition of these environments in the subsurface, and prediction of reservoir architectures and properties.
AAPG Datapages/Search and Discovery Article #90189 © 2014 AAPG Annual Convention and Exhibition, Houston, Texas, USA, April 6–9, 2014