HOLOCENE CHRONOSTRATIGRAPHIC BEACHROCK SEQUENCES AND THEIR GEOLOGIC AND CLIMATIC SIGNIFICANCE

The purpose of this study is to determine the variables involved in cementing beach sediment into beachrock. The process of cementation reflects the geochemistry and temperature conditions of the interstitial waters at the time of cementation. Since the seaward-dipping sequences reflect progressive stages of cementation, a geologic history of the shoreline may be unravelled.

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

Strontium isotopic analyses of beachrock cement reflect ⁸⁷Sr/⁸⁶Sr chronostratigraphic variation in sea water. The isotopic composition of strontium in the modern oceans is constant throughout the oceans because its residence time is ~4000 times longer than the time required for mixing (Faure 1991). Plots of radiocarbon age dates against ⁸⁷Sr/⁸⁶Sr ratios reveal patterns of strontium isotopic composition. Increasing age parallels decreasing strontium isotopic ratios. Strontium isotopic values of the beachrock cements are among the highest for Phanerozoic carbonates and point to rapid continental chemical weathering and erosion recording fast unroofing.

The oxygen isotopic composition of beachrock reflects global climate change. Following the Aisotopic temperature scale@ of Epstein et al. (1953) the oxygen isotopic changes of the prograding levels of the beachrock sequence reflect temperature changes of ambient seawater. The computed temperature excursion of the Red Sea beachrock cement implies climate decrease between the ages 7,070 ± 380yBP and 2,620 ± 230yBP, an interval of approximately 4-1/2 thousand years, during which the average Red Sea seawater temperature fell from 33°C to 17°C. This discovery is at variance with the climate change debate which involves increasing global temperatures.

Changes in d ¹³C and ⁸⁷Sr/⁸⁶Sr of the beachrock cement reflect tectonic activity recording uplift and attendant weathering of the Himalaya-Tibetan Plateu. Superimposed on this orogeny the high values for carbon and strontium isotope ratios may relate to Red Sea sea-floor spreading, related tectonic uplifting, weathering, and erosion.