A pragmatic test of the early origin and fixation of gamma-ray spectrometric (U, Th) and magneto-susceptibility (Fe) patterns related to sedimentary cycle boundaries in pure platform limestones

Abstract

In a pragmatic test conducted on vertical stratigraphic sections in Quaternary platform limestones of San Salvador Island, The Bahamas, gamma-ray spectrometric (GRS) and magnetosusceptibility (κ) data confirmed that characteristic geophysical patterns are coupled with depositional cycle boundaries. These geophysical patterns appear to develop in the early stages of diagenesis and are long lasting, because similar patterns are found both in the very young Bahamian limestones and in very old Devonian (Givetian-Frasnian) platform limestones of Moravia, Czech Republic. Because the Devonian limestones retain gamma ray and magnetic signatures similar to those seen in the Bahamian rocks, these signals are apparently resistant to changes that occur in later diagenetic alteration, including deep-burial diagenesis and 380 million years of rock-fluid interactions. Each sedimentary cycle on the Bahamian carbonate platform is marked by a terra rossa paleosol horizon that represents a lowstand emergent surface. The paleosol is typically characterized by a GRS-spike related to increased Th concentration. There is only a subtle downward infiltration of that GRS signal, but the Th signal may diffuse upward via sediment recycling. Two U-related GRS maxima are regularly developed within short distances below and above the cycle boundary. The lower anomaly reflects U enrichment in the sub-soil cementation zone, whereas the upper anomaly is related to increased U-content in the flooding beds of the next cycle. Such a combination of one Th-spike between two U-anomalies forms a distinctive tripartite GRS pattern.