Late Quaternary seismic stratigraphy and structure of the western insular shelf margin of Puerto Rico

725 km of high-resolution seismic data were collected over the insular shelf of western Puerto Rico to better understand its late Quaternary depositional and structural history. Due to low tectonic uplift rates of onshore areas in this region, well dated late Quaternary sediments and corals have only been identified in a few scattered onland localities around Puerto Rico. Seismic data from the Rio Anasco delta area of western Puerto Rico reveals four main units with characteristic stratal reflection terminations that total about 25 m in thickness. Because of a lack of well information, age estimates of these late Quaternary units are based on correlations with sea level curves derived from dated coral samples from Puerto Rico, St. Croix, and Antigua. Units include: Unit 1 - a gently folded and faulted basal section correlated to the Oliogene-early Pliocene? carbonate shelf of Puerto Rico; deeper penetration, industry MCS lines show that these rocks are deformed in a broad EW-trenching arch; Unit 2 - chaotic channel fill deposits in incisions related to the lowstand equivalent of the Rio Anasco likely formed during the Last Glacial Maximum about 25-15 ka; Unit 3 - roughly stratified deposits onlapping the top of Unit 2; these are interpreted as an estuarine facies deposited during Holocene sea level transgression; Unit 4 - highly stratified deposits related to progradation of the Anasco delta during sea level rise. The base of unit 4 is a downlap surface interpreted as a maximum flooding surface likely formed about 6 ka. East-northeast-striking faults are observed breaking the younger late Quaternary units in three separate zones off the west coast of Puerto Rico. Onland continuations of these faults have not been identified likely due to cultural overprint of natural scarps on late Quaternary floodplains.