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 pragmatictest conductedon vertical stratigraphicsectionsin Quaternaryplatform limestonesof San SalvadorIsland, The Bahamas,gamma-rayspectrometric(GRS)andmagnetosusceptibility (K) dataconfirmedthatcharacteristic geophysicalpatternsarecoupledwith depositionalcycleboundaries. These geophysicalpatternsappearto developintheearlystagesof diagenesisandare long lasting,becausesimilar patternsarefoundbothintheveryyoungBahamianlimestones andinveryoldDevonian(Givetian-Frasnian)platformlimestones ofMoravia,Czech Republic. Becausethe Devonianlimestonesretaingammaray and magneticsignaturessimilarto thoseseenin theBahamianrocks,thesesignals are apparentlyresistantto changes that occur in later diageneticalteration,includingdeep-burialdiagenesisand 380 million years of rock-fluid interactions. Each sedimentarycycle on the Bahamiancarbonateplatform is markedby a terra rossa paleosolhorizonthat representsa lowstand emergentsurface. Thepaleosolis typicallycharacterizedby a GRS-spikerelatedto increasedTh concentration. Thereis only a subtledownward infiltrationofthat GRSsignal,but theTh signalmay diffuseupwardvia sedimentrecycling. TwoV-relatedGRSmaximaareregularlydeveloped within shortdistancesbelow and above the cycleboundary. The loweranomalyreflectsV enrichmentin the sub-soilcementationzone,whereas the upper anomalyis related to increasedV -contentin the floodingbeds of the next cycle. Such a combinationof one Th-spikebetweentwoVanomaliesformsa distinctivetripartiteGRS pattern.Thex-anomaliesforma bimodalsignalthatconsistsof a narrowbut extraordinarily strongpositiveK-anomalythatiscoincidentwiththeTh-spike, andanotherspikethatisdevelopedinthe sub-soil cementationzone. Incaseswhereaburiedcycleboundaryformsthetruncatedfloorofa horizontal cave that is filledwith carbonatesediment,bothV and Th GRS peaks arebroadened. The K -curvesalso displayelevatedbut stronglyoscillating values acrossthe cave fill. The spikesare arrangedasymmetricallydownwardand the strongestspike correspondsto infiltration/cementation of the cave floor. The evidencefromthe Quaternarylimestonessuggeststhatthesetwopatterns(the tripartiteTh andV patternof the standardcycle boundary,andthebroadenedpatternrelatedto filledcaves)havean earlyorigin.In spiteoflarge inhomogeneities on cycle-boundarysurfaces,the abovegeophysicalpatternsappearto bedistinctive, differfromthenormalGRS-andx-backgroundsofplatformcarbonates,andappearstabileover the long term. This test indicatesthat these two patternsmay be useful for recognitionof exposuresurfaces/cycleboundariesvia routinepattern searches in GR, GRS and K well-logsfrom platform limestonesequencesof a wide range of ages and paleoenvironments.

Stratigraphic correlation potential of magnetic susceptibility and gamma-ray spectrometric variations in calciturbiditic facies (Silurian-Devonian boundary, Prague Synclinorium, Czech Republic)

“…The gamma-ray spectrometric (GRS) based correlations of outcrop logs have been frequently used in the last decade in the Devonian of the Czech Republic on the platform to basin formations of Moravia (Hladil et al , 2003aHladil 2002;Geršl & Hladil 2004;Bábek et al 2007, a.o. ) or Prague Synclinorium (Slavík et al 2000;Koptíková et al 2007Koptíková et al , 2008Koptíková et al , 2010.…”

Palaeoclimatic event at the Lochkovian-Pragian boundary recorded in magnetic susceptibility and gamma-ray spectrometry (Prague Synclinorium, Czech Republic)

“…In the Czech Republic this approach has been used mainly in the Devonian of the Prague Synclinorium (central Bohemia; e.g., Slavík et al 2000;Crick et al 1997Crick et al , 2001Ellwood et al 2006;Hladil et al 2010b;Koptíková 2010;Koptíková et al 2010a, b;Vacek 2010;Vacek et al 2010) and Moravosilezian Zone (Moravia; e.g., Hladil 2002;Hladil et al 2000Hladil et al , 2003aHladil et al , b, 2006Hladil et al , 2009Geršl & Hladil 2004;Bábek et al 2007;Boulvain et al 2010). Physical stratigraphic methods have been used for intrabasinal or interregional correlations as well as auxiliary data for characteristics of depositional environments and their evolution, especially in cooperation with sedimentologists and biostratigraphers.…”

Anomalous magnetic susceptibility values and traces of subsurface microbial activity in carbonate banks on San Salvador Island, Bahamas

“…The basic sampling was carried out on the Blow Hole Cave and the Gulf sections, but also on comparative vertical sections (and horizontal quadrats) in the central and northern parts of San Salvador Island (Hladil et al 2003b). This samples were obtained at 0.1-m intervals (steps) along transects.…”

“…Material of similar composition was transported to dissolution pits and small pores and fissures closely beneath, but this transport distance was not longer than several tens of centimeters, and often less. According to the geophysical logs of the vertical measured sections (Bosak et al 2002;Hladil et al 2003aHladil et al , 2003b, these Fe-rich levels are parallel with protosols and paleosols The peloids have "bleached-and-blurred" optical appearance (original slightly translucent medium dark microstructures have been changed to extremely-small-bacillar-objects (ESBO, not simply recrystallized); slightly compacted rock fabric is submerged into opaque mesh of dissolution residues, vesicular and microrhizolitic structures. B Blow Hole section, 0.18 m. Numerous and large vegeform rhizomorphs, with cavities.…”

“…The Bahama banks have been slowly but continuously subsiding during the whole Quaternary. Studies were carried out in southern part of this island where there are a significant number of previously investigated stratigraphic sections (e.g., Watling's Quarry, Blow Hole Cave, Chinese Fire Drill Cave, and the Gulf (Carew and Mylroie 1995a, 1995bBosak et al 2002;Hladil et al 2003b). The main emphasis was put on the sections at Blow Hole Cave and the Gulf (Fig.…”