Cross-Stratification in Turbidite Systems

Abstract

Cross-stratification at outcrop scale is a common structure in many depositional environments. Despite being nominally stable across a range of grainsizes and flow conditions, such small “dune-scale” cross-bedding is relatively rare in turbidites. When it does occur, it creates a distinct facies that has been identified in ancient and modern turbidite systems and as part of subsurface hydrocarbon reservoirs in cores. A key consideration with dune-scale cross-stratification is whether we can identify patterns of spatial occurrence at the system scale. Potentially, such patterns could be exploited in hydrocarbon exploration and development contexts to make large-scale predictions of sand presence and architecture. In this research, a field study is used to constrain the cross-stratification types and their characteristic facies-tract associations in a confined minibasin. Complementary metadata analysis is used to assess the patterns of occurrence of cross-stratification documented in turbidite systems more widely; this entails the categorisation of different styles and scales of cross-stratification within different turbidite systems to enable their digitisation and upload into a database.

Analysis of sixty beds containing cross-stratification in the Peira Cava basin reveals a range of cross-stratified facies types where cross-stratification may be found in association with a bed resembling a dune profile shape, or in association with a larger massive bed. Cross-stratification also occurs at the base, middle and tops of beds. This variety in facies types suggests a range of processes were involved in bedform development. Log correlations demonstrate bed continuity at kilometre scale, but with dune-scale cross-stratification facies discontinuity, suggesting bedforms in this system are not part of an extensive bedform-field. Cross-bedding is confined to proximal and medial parts of this system. However other systems, for example the Marnoso-Arenacea Fm. and the Ainsa Basin, exhibit different dune-scale bedform distribution in which cross-stratification occurs in medial-distal settings. The results to date show apparently systematic modes of occurrence of cross-strata within individual systems. It remains to be seen whether or not differences in the styles of dune occurrence between systems arise in a consistent fashion, due to combinations of specific sets of boundary conditions, or whether they are unpredictable.

AAPG Datapages/Search and Discovery Article #90323 ©2018 AAPG Annual Convention and Exhibition, Salt Lake City, Utah, May 20-23, 2018