Statistical Analysis and Stacking Patterns in Turbidite Sand Sheets (Cerro Toro Formation, Magallanes Basin, Chile)— Any Implication for Allogenic Cycles?

Abstract

Attempts have been made in the past to link thickening and coarsing upward turbidite sequences to progradation, and thinning and fining upward to retrogradation. In recent studies, more quantitative statistical analysis has been put forward and applied, but also incur some disputes about stratigraphic accretion patterns mentioned above. However, previous studies may have neglected the relationship of these patterns in sand sheets and slope channel systems, also the following factors may have been disregarded: i, if paleocurrent directions in a given succession change significantly (for example due to multiple sources), thickness and facies cycles may be not suitable as indicators of allogenic cycles; ii, erosional feature, should be regarded as an independent facies element as important as grain size change, but most previous studies have neglected; their influence on vertical thickness and facies trends may be strong where the section is intensely amalgamated; iii, confinement; if turbidity currents spread out and fill the morphologic low, vertical stacking, thickness and facies trend may have a simple and positive correlation to progradation, aggradation and retrogradation. If turbidity currents experience more compensational process, 1D successions may show a more complex relation to progradation, aggradation and retrogradation.

In this study, we consider all the factors above and test thickness and facies trends in turbidite sand sheet system in a foreland basin, with other published data as comparison. In addition to traditional analysis of descriptive statistics, we also adopt runs tests, moving average and Markov chain analysis for beds/packets (one/several depositional events respectively) or sandstones, within correlated units. The results show that runs tests are often ineffective on such scale but moving average clearly demonstrate that in more proximal parts of a highly confined basin, asymmetrical trends of thickness are common, while in distal parts more symmetrical or random trends of thickness are prevalent. The results of Markov chain cycles and frequency distribution of lithofacies also show some variation along depositional dip. In contrast to turbidite sand sheet, we built an entire 3D 3rd order slope channel model from outcrop in Rosario Formation, Maastrichtian, Mexico, to reveal the possible correlated boundaries of sequence stratigraphy in these two systems.

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