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Deltaic Facies Relationships in an Asymmetric Pull-Apart Basin, Lake Izabal, Guatemala: A Case Study From Short Sediment Cores and Geophysical Data

Abstract

Detailed sedimentological analysis of two of the largest deltaic systems in Lake Izabal, Guatemala, are studied to understand the mechanisms that control sedimentation, modern facies distribution, and stacking pattern of deltas in a tectonically active pull-apart basin. A better understanding of sedimentation in pull-apart basins is important because they hold significant hydrocarbon resources and can be important hosts of hydrocarbon source rocks. However, few studies have been conducted to establish sedimentary facies models in pull-apart systems to aid in exploration processes. To compare and contrast deltaic morphology and sedimentary facies, we used ~ 30 km of seismic reflection profiles, ~ 60 km of side-scan sonar data, and 21 sediment cores containing a total of 17 m of lake sediment collected along the delta plain, delta front, and prodeltaic settings. The cores were described at a cm scale, and their textural, compositional, and structural characteristics were recorded. In the axial margin, the Polochic delta displays a birdfoot geometry and is characterized by a moderately well-sorted sandy and silty delta plain that is relatively flat, a well-sorted silty and sandy delta front with a gentle bathymetric gradient, and a homogenous organic-rich muddy and flat prodelta. Lateral and vertical facies distributions and satellite imagery reveal a gradual downslope-fining and an upward-coarsening trend in a highly-progradational delta system. On the other hand, in the master border fault margin, the Sauce delta displays a regular lobate geometry and is characterized by a poorly sorted sandy and gravelly horizontal delta plain, a moderately sorted sandy and gravelly delta front with a steep bathymetric gradient, and a homogenous muddy and flat prodelta. Lateral and vertical facies distribution and satellite imagery reveal a rapid downslope-fining and an erratic upward-coarsening trend in a slowly progradational delta system. The variability in grain size, facies distribution, and bathymetric relief suggest that tectonics, topographic relief, and sediment texture control sedimentation in an active pull-apart basin. This study provides a modern analysis of sedimentation in an active pull-apart basin and can be used as a modern analogue for the recognition and interpretation of ancient deltaic systems in such basins.