A High-Resolution Chronostratigraphy and Rock Magnetic Record for Tortonian Shallow-Water Carbonate Sequences, Cerro de Ricardillo, Southeast Spain: Identifying Climatic and Sea-level Cycles Using Magnetic Techniques
by Paul Montgomery, Energy Technology Company; Evan K. Franseen, Kansas Geological Survey; Robert H. Goldstein, University of Kansas; and Niall K. Toomey, University of Kansas
Well-exposed Upper Miocene shallow-water carbonate sequences in the Cerro de Ricardillo area of southeastern Spain provided the opportunity to produce a high resolution chronostratigraphy based on magnetic polarity and magnetic susceptibility (MS), and to investigate the origin of the MS signal and its utility for aiding in understanding controls on deposition.
The study focused on the three lowest sequences (DS1A, DS1B, DS2A), corresponding to chronozones C4An-C3Br3 (8.8-7.4 Ma), that were deposited on irregular volcanic basement paleotopography. DS1A, DS1B and lowermost DS2 consist primarily of relatively coarse-grained mollusc-bryozoan packstones and volcanic detritus facies consistent with temperate conditions. DS2 comprises 12 shoaling-upward cycles and uppermost DS2 strata contain the first chlorozoans indicating a shift towards tropical conditions.
MS profiles from two time-equivalent sections 0.5 km apart produced MS cycles based on changes in magnetic mineralogy that matched closely with sedimentary cycles within sequences identified and physically traced in the field, including meter(s) scale DS2 cycles. In general, lower MS reflect superparamagnetic grain-sizes and higher concentrations of hematite coincide with subaerial exposure surfaces and tops of shallowing-upward cycles. Higher MS indicates magnetite and correspond with deposits immediately above subaerial exposure surfaces and the base of shallowing-upward cycles. A high degree of correlation between MS profiles and the field-based stratigraphy and to chronostratigraphically constrained relative sea-level curves derived from this area and surrounding areas demonstrates the utility of MS profiles for high-resolution correlation, and for aiding evaluation of controls on sequences and cycles, which appear to reflect climate changes and relative sea-level fluctuations.