DELTAIC SEDIMENTATION IN AN ASYMMETRIC PULL-APART BASIN, LAKE IZABAL, GUATEMALA: NEW CONSTRAINTS FROM SHORT SEDIMENT CORES AND GEOPHYSICAL DATA

A detailed sedimentological analysis of two of the largest deltaic systems in Lake Izabal, Guatemala, were studied to understand the mechanisms that control facies distribution and stacking pattern of deltas in a tectonically active pull-apart basin. To compare and contrast deltaic morphology and sedimentary facies, we used ~ 30 km of seismic reflection profiles, ~ 60 km of side-scan sonar data, satellite imagery, 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 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 sedimentological, bathymetric and imagery data indicate that the Polochic delta is a fine sand river-dominated system, whereas the Sauce delta is a gravelly sand river-dominated and wave-influenced system and can be classified as a Gilbert-type delta. The variability in grain size, facies distribution, and bathymetric relief suggest that the tectonic setting and the proximity of the source area are primary controls on the two deltaic systems. This study provides a better understanding of deltas that develop in an active tectonic setting that can be used as a modern analogue for the recognition and interpretation of ancient deltaic systems.