Ladislav Slavík

Early diagenetic origin and persistence of gamma-ray and magnetosusceptibility patterns in platform carbonates: comparison of Devonian and Quaternary sections

Abstract Gamma-ray logs from boreholes in the Middle–Upper Devonian carbonate platform of Moravia display tripartite anomalies at locations, where lithological and biostratigraphic data suggest the occurrence of 4th order sedimentary cycle boundaries. Further, where sedimentary boundaries have been changed by later development of caves (usually phreatic caves changed to unroofed caves––erosion), the carbonate infillings in these corroded cycle boundaries are marked by another pattern that consists of a smooth symmetrical peak on gamma-ray activity in vertical section. The details procured using gamma-ray spectrometric and magnetosusceptibility methods suggest that the upper peak of the tripartite pattern corresponds solely to uranium concentration (flooding surface). The middle peak is marked by a thorium signal and a magnetosusceptibility response from paramagnetic minerals (paleosols). The lower peak corresponds to trapped uranium and microbial magnetite in cemented rock pores (originally dysoxic microenvironments in calcite). The boundaries marked with filled caves display only one broad and symmetrical uranium-related peak, and the thorium peak that is roughly similar to that seen at normal boundaries, but it is shifted slightly downward. At boundaries with caves the magnetosusceptibility peaks are shifted downward considerably, and may even occur within the underlying host rock. The question of whether these patterns are a primary imprint of early diagenetic influences or a much later redistribution that originated during pressure solution and cementation, was answered by study of Late Quaternary sections on San Salvador Island, Bahamas. This pragmatic test on young carbonate sediments confirmed the early origin and fixation of these geophysical patterns.

The graptolite, conodont and sedimentary record through the late Ludlow Kozlowskii Event (Silurian) in the shale-dominated succession of Bohemia

The shale-dominated hemipelagic succession exposed in the southwestern part of the Prague Synform preserves the most complete Ludfordian graptolite record so far encountered from peri-Gondwanan Europe. Four graptolite biozones – the Neocucullograptus inexpectatus , Nc. kozlowskii , Pseudomonoclimacis latilobus–Slovinograptus balticus and Pristiograptus fragmentalis biozones – are recognized in the middle and late Ludfordian, between the Bohemograptus tenuis Biozone and the base of the Pridoli Series. Conodont occurrences are restricted to scattered limestone beds, but enable tentative integration of the graptolite and conodont biozonal schemes. Particular attention was paid to faunal and sedimentary changes and the carbon isotope record across the middle Ludfordian Kozlowskii extinction Event. The Kozlowskii Event caused the almost simultaneous extinction of graptolites with ventrally curved rhabdosomes. The genera Bohemograptus , Polonograptus and Neocucullograptus , along with Pseudomonoclimacis dalejensis , disappeared from the fossil record. The offshore conodont fauna recorded in the section was not strongly affected and similarly the pelagic orthocerids and nektonic Ceratiocaris passed unaffected through the extinction interval. The abundant and widespread pelagic myodocopid ostracod Entomis , however, became extinct. The late Ludfordian graptolite recovery gave origin to a novel fauna of Pridoli type from taxa that emerged or just reappeared above the Kozlowskii crisis. In Vseradice and elsewhere in the Prague Synform, the recovery, manifested by the appearance of Pseudomonoclimacis latilobus and Slovinograptus balticus , closely postdates the end of the isotope excursion but pre-dates the first appearance of the conodont index ‘ Ozarkodina ’ snajdri . Here the graptolite recovery was delayed relative to the recovery of the benthic fauna. A canalized intraformational limestone conglomerate corresponds with a gap in the sedimentary record above the Kozlowskii extinction and just below the graptolite recovery. The benthic faunas from the conglomerate matrix and pebbles permit correlation with the shallower part of the basin indicating a distinct fall in sea-level. The present data demonstrate the coincidence of the graptolite crisis with benthic faunal change and eustatic fall in sea-level manifested by facies change and the carbon isotope excursion. Polonograptus chlupaci sp. nov., from the Nc. kozlowskii Biozone, is described and several other graptolite taxa are redescribed.

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

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.The κ-anomalies form a bimodal signal that consists of a narrow but extraordinarily strong positive κ-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 κ-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 κ-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 κ well-logs from platform limestone sequences of a wide range of ages and paleoenvironments.

Petrophysical record of the Late Silurian shallow-water carbonate facies across the Lau Event (Prague Synform, Czech Republic) and dynamic time warping alignment of the magnetic susceptibility logs

Abstract High-resolution petrophysical logging magnetic susceptibility measurements (MS) and gamma-ray spectrometry (GRS), supplemented by rock magnetic measurements (anhysteretic and isothermal remanent magnetizations (ARM/IRM), frequency-dependent MS (FDMS)) and MS logs alignment using the dynamic time warping (DTW) algorithm, were constrained by upgraded conodont biozonation across the Lau Event interval (LEI). Two sections (Pozary and Muslovka) representing shallow-water facies in the Prague Synform were studied. Conodont data and DTW alignment show that Muslovka represents a less complete succession when compared to the Pozary. DTW alignment of the MS logs shows perfect match of the logs and detected six prominent and several minor gaps. MS logs show elevated values and triple peaks at both sections across the LEI which start in the uppermost part of the siluricus Biozone. Magnetite was identified as the main carrier of the MS signal at both sections using ARM and IRM. FDMS revealed occurrence of superparamagnetic (SP) particles at both sections in all selected samples. Furthermore, a higher contribution of SP particles to the MS signal is recorded below the Lau Event interval in the Pozary section. GRS logs show depletion in U concentrations across the LEI at both sections.

Integrated stratigraphy of the Ludfordian in the Prague Synform

The stratigraphic correlation of the Ludfordian in the Prague Synform is summarized using refined biostratigraphic data from updated conodont and graptolite faunal records. The seven graptolite and seven conodont biozones and faunal intervals are correlated. The biozonations are then integrated with the generalized eustatic and carbon isotope curves. Levels of major extinction and radiation events are plotted against an integrated and refined Ludfordian stratigraphy of the Prague Synform. The proposed correlation chart is a basis for further high-resolution correlation of that region.

Refining the late Silurian sea-level history of the Prague Syncline—a case study based on the Přídolí GSSP (Czech Republic)

A 50-m-thick section in the Požáry quarry, Prague Syncline (Czech Republic) spanning the upper Silurian (uppermost Ludlow and Přídolí) to the lowermost Devonian (Lochkovian) has been studied using sedimentological and physical stratigraphical (gamma-ray spectrometry) methods combined with conodont biostratigraphy. Conodont data demonstrate the presence of local conodont biozones: “Ozarkodina” crispa (uppermost Ludlow)—Zieglerodina zellmeri (base of Přídolí)—Zieglerodina ivochlupaci—Delotaxis detorta—“Ozarkodina” eosteinhornensis s.s.—Zieglerodina klonkensis—Icriodus hesperius-optima (lowermost Lochkovian). The studied section represents a transgressive–regressive facies succession characterized by a transition from distal calciturbidites deposited in a distally steepened carbonate platform during transgression and sea-level highstand (crispa to lower ivochlupaci zones) to mixed calciturbidites/coarse-grained bioclastic limestones (falling-stage systems tract, upper ivochlupaci to lower klonkensis zones). The upper part of the succession consists of bioclastic limestones corresponding to a subtidal setting between storm-wave and fair-weather wave base (lowstand systems tract in upper klonkensis and hesperius-optima zones). This interpretation corresponds to the late Silurian global sea-level pattern as recorded in other regions. Thus the depositional system is interpreted as mostly driven by eustasy with short-lived periods of influence from local tectonics.