Frequency Distribution of Bed Thickness in Slope-Channel Fills: New Insights From the Cretaceous Tres Pasos Formation, Chile

Abstract

The frequency distribution of turbidite bed thickness has been interpreted to record information on flow dynamics, initial sediment volumes, and depositional setting; it is also an important influence on hydrocarbon reservoirs. Power laws are commonly used to describe these thickness distributions, and have been linked to initial source volumes associated with earthquakes with comparably distributed magnitudes. Some turbidite successions exhibit bed-thickness distributions that are better described by a log-normal mixture model: a model in which two log-normal distributions are associated with beds that exhibit a certain grain size or sedimentary structure at the base. We assess the distribution of turbidite bed thickness in slope-channel strata from the Cretaceous Tres Pasos Formation, southern Chile, and compare with published data from channel (Paleogene Tarcau Sandstone, Romania) and basin-floor lobe strata (Miocene Marnoso-Arenacea Formation, NW Italy). The Tres Pasos dataset comprises >10 individual channel elements (typically 20 m thick and 250-300 m wide), which can be placed in the context of a well-constrained stratigraphic framework. This framework provides a unique opportunity to consider the distribution of thickness for > 3000 turbidite beds, both in general channelized settings and in terms of their intra-channel positions (i.e., the axis, off-axis, or margin of channel fills). A log-normal mixture model best describes the distribution of turbidite bed thickness within slope-channel fills of the Tres Pasos Formation, and is comparable to that documented for channelized and lobe strata in previous studies. Variations in the distribution of bed thickness, in addition to other sedimentological criteria (e.g., grain size, bed type and depositional facies), offer insight into depositional processes and setting. For example, differences in the bed-thickness distribution from axis, off-axis and margin channel fills highlight the influence of differential erosion and deposition within the channel, and associated channel relief, upon the preserved channel-fill strata. Quantifying bed thicknesses in channelized strata improves our understanding of these systems, and facilitates effective modeling of fluid storage and flow within subsurface reservoirs. Further, our observations support previous studies suggesting that turbidite bed thickness does not always follow a power-law model, and thus cannot be directly related to earthquake magnitudes.

AAPG Datapages/Search and Discovery Article #90291 ©2017 AAPG Annual Convention and Exhibition, Houston, Texas, April 2-5, 2017