Jindrich Hladil

Geophysical records of dispersed weathering products on the Frasnian carbonate platform and early Famennian ramps in Moravia, Czech Republic: proxies for eustasy and palaeoclimate

Abstract An exceptionally high sea level in Frasnian times (early Late Devonian) changed many peneplained, cratonised and tectonically undisturbed continental margins of tropical belts into carbonate platforms which became covered by thick banks of limestone. The stages marked by very pure carbonates developed during partial sea-level highstands since, when the flooding reached its maximum, the shoreline was distant and conditions for reef growth were good. An increased extension of emerged, flat, coastal areas was coupled with an opposite situation, when oscillating sea level fell to its momentary minimum and the accommodation space available for accumulation of carbonate material was low. Subaerially exposed surfaces were subject to weathering and the products were dispersed by aeolian or aquatic means to form layers when these weathering materials were trapped in carbonates. This overall scenario relating to the trapping of weathering products in pure carbonates was tested at geographically isolated platform with a stable and only slowly subsiding basement (Eastern Moravia, Czech Republic). The amounts of weathering products dispersed in limestone were documented using the combined methods of γ-ray logging, γ-ray spectrometry and magnetic susceptibility measuring, i.e. according to their radioactivity (K, Th and U) and magnetism (Fe). Six third-order Frasnian eustatic cycles have been inferred from the data, with an estimated period of 1 Ma. The Late Frasnian and especially the Frasnian/Famennian (F/F) transition brought the total destruction or at least a strong overlay on this hypothetical background setting. The record of strong sea-level fluctuations in the F/F interval is shown in strong oscillations and offsets related to pronounced hiatuses and flooding surfaces. Such rapid sea-level fluctuation is typical for glacioeustatic control, although the present estimates of the maximum amplitudes are larger than realisable by glaciation events and may be explained in combination with tectonoeustatic models. Comparison of sections between the centre and periphery of the platform substantiates the concept that oscillating Late Frasnian shorelines were rapidly prograding. Very low sea-level stages of early Famennian times changed the depositionary systems from extensive platforms to narrow carbonate ramps which were positioned out and down on the extinct reef bank slopes. The central parts of the Eastern Moravian and also other Frasnian platforms in the world were not normally covered by F/F sediments, the transgressive onlaps above any crystalline highs were not observed and therefore the drowned Frasnian reefs on their periphery, although emphasised by many geologists, may reflect sliding or tectonic subsidence on the margins of platform blocks rather than eustatic rise of sea level.

Trends in the development and cyclic patterns of middle and upper devonian buildups

SummaryA comprehensive review of Middle and Upper Devonian reefs, based on about 350 papers describing the reef development within 44 areas all over the world, resulted in the recognition of four reef megacycles (Upper Eifelian; Givetian; Uppermost Givetian to Frasnian; Upper Frasnian to Lower Famennian). These megacycles are characterized by the occurrence of typical reef types and by characteristic microfacies associations. Twelve microfacies associations can be distinguished using lithological, paleontological and microfacies criteria; each microfacies association includes several standard microfacies types.The evolution of the Middle and Upper Devonian buildups is illustrated by a sequence of nine stages (Fig. 2): Semilagoonal stage with pioneer bentic communities; Stage with coral and brachiopod banks; Stage withAmphipora banks and flat mud mounds with stromatoporoids; Stage with stromatoporoid-coral banks occurring within scattered patch reefs; Lagoonal stages, generally regressive; Stage with shoals or barrier reefs (including a fore-reef development as well as reef margins with a back-reef facies); Reef cap stage indicating the end of reef growth; Stage of drowned reef surfaces with carbonate sands and gravels; Stage characterized by the beginning of a non-reefal sedimentation (mainly nodular limestones).Differences between Devonian and recent reefs include the frequency of the standard microfacies types (Fig. 5) as well as different tectonic settings of the reefs (about 80% of the reefs were formed in the area of rapidly subsiding platforms or quasiplatform blocks within miogeosynclines; the angle of the platform slopes was smaller than in recent examples).

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.

Are there turbidites in the Silurian/Devonian boundary stratotype? (Klonk near Suchomasty, Barrandian, Czechoslovakia)

SummaryA discussion arose in 1977 regarding the nature of the Silurian/Devonian boundary bed at Klonk, and the beds below and above it. Present revision of the stratotype sequence found that most clayey limestones display a multiple and composed rhythmic arrangement of laminae. Deposition of pelagic particles, effects of traction bottom currents, and turbidite inputs are distinguishable, however, the latter are rare. Semilithified surfaces and hardgrounds were found. The boundary bed No. 20 consists of several laminated rhythms. The Devonian base, marked by first occurrences ofMonograptus uniformis corresponds to a semilithified surface, a break in deposition for several tens to hundreds of years, and a change in direction of bottom currents. A moderately rippled set at the Devonian base is only about 1 cm thick and passes again into the horizontal laminated rhythms. The deposition of the boundary bed lasted about 1.2 to 2.0 Ka. It cannot be explained as a turbidite.

Sedimentology and orientation of tentaculite shells in turbidite lime mudstone to packstone; Lower Devonian, Barrandian, Bohemia

ABSTRACT A turbidite origin of upper Pragian lime mudstone is proposed on the basis of imbrication and graded bedding in the lower parts of beds and the abundance of calcisiltite components. Deposition of lime mud and calcisiltite dominated the middle parts of the turbidite beds. This muddy part of turbulent gravity surges settled on the submarine slope as a dense suspension. Downslope sliding continued after deposition from sediment gravity flows as dewatering occurred and the sediment underwent plastic deformation. The spatial orientation of embedded tentaculite shells is unusual, but corroborates the sediment-gravity-flow origin of the beds. The apices of narrow conical shells show preferred positions of ca. 50° upward or ca. 50° downward, with upward directions more common (63.5 ); the shells avoid both bedding-parallel and bedding-perpendicular orientations. Double-cone envelopes arranged according to a subhorizontal mirror plane are the basic orientation pattern. This double-cone pattern of movement of narrow-cone particles was successfully simulated by simple experiments in turbulent water. The steep inclination of shells reflects rapid solidification of dense turbulent mud. Solidification was probably intensified by later sliding and thrusting shears during the early plastic stage of sediment deformation. Azimuth directions show only slight peaks parallel and perpendicular to the direction of downslope propagation of the gravity flow. The complex distribution of tentaculitid shells was analyzed by factor analysis, which yielded four types of orientation patt rns that indicate a prevailing orientation of gravity flows towards the SSW and SSE.

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.

Magnetic susceptibility application : A window onto ancient environments and climatic variations: Foreword

Abstract Magnetic susceptibility (MS) is a powerful tool, which is being applied increasingly on sedimentary rocks to constrain stratigraphic correlations, or as a palaeo-environmental or palaeo-climatic tool. The origin of the magnetic minerals responsible for the variations in MS can be linked to various phenomena such as detrital inputs, pedogenesis, bacterial precipitation or diagenesis. Therefore, it is critical to improve our knowledge of the origin of the MS signal in order to apply it for correlations or as a proxy. Here, we present a synthesis of the techniques that can be applied to get a better understanding of the origin of the MS signal, through comparison with other palaeo-environmental proxies, through magnetic measurements or through dissolution and direct observation of the extracted minerals. We also propose an overview of the different techniques applied in order to use MS as a correlation tool, and we show various examples of successful applications of MS as a recorder of change in past sea-level and climate. We also present the main results and activities of the IGCP-580 project ‘Application of magnetic susceptibility as a palaeo-climatic proxy on Palaeozoic sedimentary rocks and characterization of the magnetic signal’.

Stromatactic patterns formation in geological sediments: field observations versus experiments

We demonstrate a novel purely hydrodynamic concept of formation of stromatactic cavities in geological sediments, originated by Hladil (2005a,b). First, the characteristic features of these cavities are described, as for their geometry and occurrence in the sedimentary rocks, and the several existing contemporary concepts of their formation are briefly reviewed. Then the new concept is introduced, and laboratory experiments described that were designed to validate it. Finally, the result obtained are presented and discussed, and the prospect for the future research is outlined. Note that the stromatactic patterns are three-dimensional cavities which are formed inside the rapidly thickening suspension/sediment. These are not the surface-related patterns like ripples or dunes.

Late Variscan remagnetization of Devonian carbonates in the Moravo-Silesian zone (Czech Republic): implications for dating tectonic deformation

The Subandean Basins of South America extending from Trinidad to Tierra del Fuego have been the object of intensive exploratory activities (Fig. 1). The largest amount of hydrocarbons discovered during the last 30 years in these basins was found in complex structural terrains. A total of 59 Billion Barrels of Oil Equivalent (BBOE) have been discovered in areas affected by compressional tectonics. Of these basins, the largest discoveries are in the Furrial Trend of Venezuela (24 BBOE), followed by the Chaco area in Bolivia and Argentina (13 BBOE), the Llanos Foothills of Colombia (4.4 BBOE), and the Madre de Dios Basin of Peru (4.2 BBOE).