STRATIGRAPHIC ARCHITECTURE AND STRUCTURAL AND SEDIMENTOLOGICAL EVOLUTION OF AN ASYMMETRIC PULL-APART BASIN, LAKE IZABAL, GUATEMALA

The Lake Izabal Basin in eastern Guatemala is a pull-apart basin that developed along the Polochic Fault which represents the western, inland segment of the North America-Caribbean plate boundary. The basin contains a thick sedimentary fill that can be used to constrain the tectonic and structural evolution of this poorly-studied region. We integrated a set of multichannel seismic reflection profiles, well and gravity data, and newly acquired shallow seismic, short sediment core, and outcrop data to study the stratigraphic architecture, structural evolution, and basin-fill history of the basin. Geological mapping, combined with outcrop and well data, suggest that the basin developed in the Miocene. Seismic profiles and gravity data reveal a highly asymmetrical basin, with an approximate sedimentary fill of 4 km along the northern side of the basin that thins significantly to the south. This asymmetry is a result of transform-normal extension in the basin. Evaluation of outcrop and well data suggest that the basin was initially filled by shallow marine and fluvial sediments with a transition to lacustrine sediments in the last 1500 m. Seismic mapping and stratal termination patterns indicate that the basin has grown through time and that the depocenter has shifted westward close to 75-80 km in 10 My. The sedimentary fill displays a clear progradational pattern that has migrated westward in response to the migration of the depocenter. The basin today is occupied in its eastern part by Lake Izabal, the largest lake in Guatemala. The western part, on the other hand, is occupied by a large wetland and by the Polochic delta plain. Shallow seismic data and short cores obtained from the basin center suggest that Lake Izabal underwent an episode of contraction in the recent past indicated by a significant change in the sedimentological characteristics of the shallow sediments as well as a shallow seismic surface that appears to be erosive. The recent fill has healed this erosive surface, creating a shallow, 15 m deep lake with a flat bottom. This study provides a comprehensive examination of a diverse dataset that can help understand the evolution of the Lake Izabal pull-apart basin. Such work is needed to constrain the tectonic and structural history of an understudied continental strike-slip plate boundary.