Resumen de 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
Jindrich Hladil, Pavel Bosak, Ladislav Slavik, James L. Carew, John E. Mylroie, Milan Gersl
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 (k) 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.
The k-anomalies form a bimodal signal that consists of a narrow but extraordinarily strong positive k-anomaly that is coincident with the Th-spike, and another spike that is developed in the sub-soil cementation zone. In cases where a buried cycle boundary forms the truncated floor of a horizontal cave that is filled with carbonate sediment, both U and Th GRS peaks are broadened. The k-curves also display elevated but strongly oscillating values across the cave fill. The spikes are arranged asymmetrically downward and the strongest spike corresponds to infiltration/cementation of the cave floor. The evidence from the Quaternary limestones suggests that these two patterns (the tripartite Th and U pattern of the standard cycle boundary, and the broadened pattern related to filled caves) have an early origin. In spite of large inhomogeneities on cycle-boundary surfaces, the above geophysical patterns appear to be distinctive, differ from the normal GRS-and k-backgrounds of platform carbonates, and appear stabile over the long term. This test indicates that these two patterns may be useful for recognition of exposure surfaces/cycle boundaries via routine pattern searches in GR, GRS and k well-logs from platform limestone sequences of a wide range of ages and paleoenvironments.
