Cyclostratigraphic analysis of the early Cretaceous laminated limestones of the Araripe Basin, NE Brazil: Estimating sedimentary depositional rates

Abstract

By the Early Cretaceous, the Araripe Basin (Northeastern Brazil) is interpreted as having gone through a phase of tectonic quiescence, during which rates of sediment accumulation were mostly modulated by climatic variations. The Crato Member (Santana Formation, upper Aptian of Araripe Basin) consists of gray calciferous mudstone and parallel-stratified micritic limestone deposited in a lacustrine environment. The present study applied the principles of cyclostratigraphic analysis in the uppermost carbonate unit of the Crato Member, which comprises laminated limestone displaying a characteristic cyclic alternation of pale to dark laminae, in order to estimate sediment accumulation rates by relating the depositional record with astronomical cycles of known periodicity. An 8.5 m thick limestone succession was analyzed in order to build a time series by using two distinct data sources, namely spectral gamma ray readings and color variation chart. Additionally, petrographic and organic composition analyses were performed to investigate if environmental changes have affected those parameters. Results indicate that darker laminae reflect periods with concentration of sulfite related to relatively anoxic water conditions. The time series analysis allowed for an identification of two different astronomical cycles: (1) Milankovitch cycles, ranging from 18.6 ky to 39.4 ky, resulting in an estimated depositional rate of 3.25 cm/ky, with a total of 261 ky for the deposition of the whole studied interval; (2) Suess solar cycles, ranging 180–200 years, with an estimated depositional rate of 3.3 cm/ky, and a total of 257 ky for the deposition of the same interval. Therefore, this multidisciplinary approach led to the identification of Milankovitch and Suess high frequency solar cycles, based on which it was possible to determine the rate of deposition as well as the time span involved in it.