Late Quaternary seismic stratigraphy and structure of the western insular shelf margin of Puerto Rico
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The modern insular shelf and slope of western Puerto Rico is characterized by reef carbonate and siliciclastic sedimentary environments controlled by the complex interplay of tectonic arching and faulting, vigorous erosion of the elevated island of Puerto Rico, fluvial incision and sedimentation pulses, reef growth, and rapid late Quaternary eustatic sea-level changes. For this study, I interpret 725 km of high-resolution, single-channel seismic lines that were collected over the western insular shelf and slope by the RV Isla Magueyes in 2000 to better understand tectonic, erosional, and eustatic controls on late Quaternary history. The seafloor geology of the western shelf and slope area of Puerto Rico has been well studied and mapped mainly from grab and short cores collected by geoscientists at the U.S. Geological Survey and at the University of Puerto Rico at Mayagü̈ez. Because of low tectonic uplift rates from the western coast of Puerto Rico, the onland late Quaternary sedimentary history is not recorded in Quaternary coastal outcrops. The results of my seismic interpretation and correlation with multi-channel seismic data collected in 1972, reveal four main units defined by characteristic stratal reflection terminations and seismic facies. These units include: Unit 1 (basement) - a gently folded and faulted basal section correlated to the Oligocene-early Pleistocene carbonate shelf of Puerto Rico; deeper penetration, industry multi-channel seismic lines show that these rocks are deformed in a broad east-west-trending arch; Unit 2 (lowstand systems tract, LST) - chaotic channel fill deposits filling deep (~ 30 meter) incisions formed during the Last Glacial Maximum about 20 ka; Unit 3 (transgressive system tract, TST) - poorly stratified deposits truncating the top of Unit 2 and deposited during early Holocene transgression of the shelf margin; Unit 4 (highstand system tract, HST) - late Holocene, highly stratified deposits related to aggradation as the Holocene transgression began to slow. The base of unit 4 is a downlap surface interpreted as a maximum flooding surface likely formed about 8 ka. East-northeast-striking faults are observed that offset the late Quaternary units in three separate zones off the west coast of Puerto Rico. Because of a lack of wells and long cores from the shelf and slope area, age estimates for the four units are based on correlations with sea-level curves derived from dated coral samples in the Caribbean and western Atlantic region. All four units are deformed by faulting that should be considered active and possibly hazardous for the rapidly developing west coast of Puerto Rico. In one area, a large, late Holocene-slump (~ 0.016 km³) is mapped using seismic, sidescan sonar and bathymetric data. Onland continuations of these faults are likely, but have not been identified due to cultural overprint of natural scarps on late Quaternary floodplains.