SEDIMENTARY PROCESSES AND FACIES IN AN ACTIVE PULL-APART: INTERPRETATIONS OF SHORT-SEDIMENT CORES AND GEOPHYSICAL DATA FROM LAKE IZABAL BASIN, GUATEMALA

Detailed sedimentological analysis of short sediment cores and geophysical data acquired in Lake Izabal, Guatemala, are studied to understand the mechanisms that control sedimentation and modern facies distribution in a tectonically active pull-apart basin. Most of the knowledge about sedimentation in pull-apart basins is derived from now exhumed systems (e.g., Ridge basin, southern California) and from a few studies of modern pull-apart basins (e.g., Lake Hazar basin). This study provides a modern analysis of sedimentation in a tectonically active pull-apart basin that can be used as a modern analogue for the recognition and interpretation of ancient pull-apart basins. To compare morphology and sedimentary facies, we used ~ 30 km of seismic reflection profiles, ~ 60 km of side-scan sonar data, and 25 sediment cores containing more than 30 m of lake sediment. The cores were described at a cm scale, and their textural, compositional, and structural characteristics were recorded. The results indicate high variability in grain size, lithofacies distribution, and bathymetric relief between sediments deposited in the axial and master border fault margin of the basin. In the axial margin, lithofacies are composed of moderately well sorted sandy and organic-rich silty deposits with a gentle bathymetric gradient. Lateral and vertical facies distributions and satellite imagery reveal a gradual downslope-fining and an upward-coarsening trend in a highly-progradational system. By contrast, lithofacies in the master border fault margin are composed of poorly to moderately sorted sandy and gravelly deposits, with a steep bathymetric gradient. Lateral and vertical facies distribution and satellite imagery reveal a rapid downslope-fining and an erratic upward-coarsening trend in a slow progradational system. Variability in grain size, facies distribution, and bathymetry in Lake Izabal is used as a modern analogue to improve the interpretations of ancient pull-apart basins and other active pull-apart basins.