ABSTRACT: Rhythm of Beachrock Levels in Contrasting Plate Settings: Rifting Red Sea and North American-Caribbean Plates

FRIEDMAN, GERALD M., Brooklyn College and Graduate School of the City University of New York, and Northeastern Science Foundation, Troy, NY

Sequences of beachrocks have been compared from the northern Red Sea, the North American plate (north shore of Puerto Rico), and an island of the Caribbean plate. In all these areas the beachrocks exhibit regular beds dipping gently seawards.

The Red Sea beachrocks increase in age progressively from 2,620 plus or minus 230 yBP to 3,570 plus or minus 170 yBP, to 6,220 plus or minus 280 yBP, and to 7,070 plus or minus 380 yBP. Comparison with the seawater ⁸⁷Sr/⁸⁶Sr reference section demonstrates that the carbonate cements precipitated from waters that are in strontium isotopic equilibrium with the global ocean. By contrast the beachrocks from the North American and Caribbean plates are much younger (< 100 to 2,520 plus or minus 120 yBP), and the strontium isotopic composition of the North American samples is at variance with the strontium isotopic reference curve.

The stable isotopic compositions of the carbonate materials of the Red Sea beachrocks increase progressively with time (d13C from +1.9 to +2.2, +2.3, +3.2, and +3.5 and d¹⁸O from -3.5 to -1.9, -0.7, -0.1, and +0.6) reflecting progressive climatic and/or salinity changes of the Red Sea waters. By contrast the stable isotopic compositions of the carbonate material of beachrocks of the North American plate studied exhibits a random pattern.

The four to five levels of beachrock in the three studied areas represent a rhythm of sea-level fluctuations and/or tectonic activity, including major earthquakes. Earthquakes are indicated especially for the North American plate, where at least three levels of beachrock were generated during the last 100 years.

Search and Discovery Article #90907©2000 AAPG Eastern Section Meeting, London, Ontario, Canada