Bioclastic Beds Research Articles

Early diagenesis driven by widespread meteoric infiltration of a Central European carbonate ramp: A reinterpretation of the Upper Muschelkalk

Meteoric diagenesis of carbonate ramps is often difficult to interpret and can commonly be confused with other coinciding diagenetic processes. The Middle Triassic Upper Muschelkalk of Switzerland provides an insightful case in which the effects of several overprinting diagenetic environments, including matrix dolomitization, can be clearly unravelled. Previous studies suggested that diagenesis took place in connate marine waters, with later meteoric waters being invoked to explain recrystallization of dolomite. In this study, diagenetic analyses (C–O stable isotope ratios, thin-section point counting, cathodoluminescence and UV-fluorescence microscopy) of calcitic bioclastic samples have revealed that early diagenesis (pre-stylolitization) and the accompanying porosity evolution did not occur exclusively in the presence of marine fluids. Five sequential stages of diagenesis have been identified: marine, shallow burial, mixing-zone, meteoric and dolomitization. Marine diagenesis induced precipitation of bladed and inclusion-rich syntaxial cements that fluoresce strongly under UV-light. Both cements account for a mean 7.5vol% reduction in the porosity of bioclastic beds. Shallow burial diagenesis likely induced mouldic porosity and associated fluorescent dog-tooth cementation. Based on light oxygen isotope and elevated strontium isotope ratios, matrix aragonite–calcite neomorphism is interpreted to have occurred in a mixture of marine and meteoric fluids. The combination of shallow burial and mixing-zone processes reduced porosity on average by 4.8vol%. Evidence for subsequent meteoric diagenesis is found in abundant dog-tooth and blocky calcite cements that have mean δ18OVPDB of −9.36‰ and no signs of recrystallization. These meteoric cements reduced porosity by a further 13.4vol%. Percolation of meteoric water through the ramp was driven by hydraulic gradients on an adjacent basement high, which was exposed by a cycle of early Ladinian regressions. Following meteoric diagenesis the Upper Muschelkalk was dolomitized by refluxing brines. This complex history of diagenesis resulted in moderate porosities in dolomitized rocks (up to 20%), and low porosities (<5%) in calcitic bioclastic beds. These results are used to show that the present-day reservoir properties of non-dolomitized carbonate rocks, particularly bioclastic beds, can be largely attributed to early diagenetic processes. Thus, knowledge of the early diagenetic history and its regional controls provides a means to predict reservoir properties over wide areas between and beyond well sites.

Island slopes and jumbled shell beds

Shell beds pose questions of provenance, but determining the degree of pre-depositional transport of bioclasts can be speculative. Bioclastic beds deposited on a broad, shallow continental shelf may consist of a minimally transported accumulation. In contrast, islands may have steep shelves and accumulate disharmonious bioclastic assemblages in deeper water. The latter are a signature of the stratigraphic record on many islands. Herein, we contrast differing examples of jumbled shell beds preserved around islands. The Lower Permian Mengkarang Formation of Sumatra preserves deep-water facies at the base of a volcanic section. Nektonic cephalopods overlie a fine-grained ash containing a diverse association of terrestrial pollen. The Lower Miocene Montpelier Formation of Jamaica yields allochthonous packstones and slide blocks of reef limestone embedded in chalk; the ichthyofauna indicates deposition in ≥200 m water depth. Abundant evidence that deposition occurred in a deep-water turbidite basin flanking a volcanic island is found in the Middle Miocene Grand Bay Formation of Carriacou, the Grenadines. Diverse, deeper water taxa are preserved in association with land snails, hermatypic scleractinians, algal balls and planktic pteropods. The famous Upper Pliocene Bowden shell beds, Jamaica, preserve terrestrial, shallow-water, deep-water and planktic taxa.

Blackened bioclasts and bituminous impregnations in the Koněprusy Limestone (Lower Devonian), the Barrandian area, Czech Republic: implications for basin analysis

Carbonate reef talus facies of the Koněprusy Limestone (Pragian, Lower Devonian, Barrandian) locally exhibit widespread impregnation by organic matter resulting in a partial to complete blackening of the limestones. Two contrasting types of impregnation are recognized: blackening of individual carbonate fossils and bioclastic layers within the limestone originated very early during diagenesis. The blackening is due to finely dispersed organic matter and possibly some iron sulphides and clay minerals that selectively adhered to the outer layers of corals, bryozoans, and crinoid fragments, leaving other fossils unaltered. These darkened fossils are similar to black pebbles—i.e., reworked, dark to black limestone clasts and bioclasts that are known to occur exclusively in shallow-water zones of both ancient and modern carbonates. The alteration of fossil fragments may have taken place in very shallow-water environments, possibly those of saline and reducing back-reef lagoons or supratidal-intertidal zones, with organic matter being derived from decayed algae and microbes, or early vascular terrestrial plant material. Following the coloration, the blackened fossils were removed from their original position by waves or storms and transported into relatively deeper-water reef slope settings to form graded, “salt-and-pepper”-colored bioclastic beds. The presence of blackened fossils in the carbonate succession may point to episodic emergence and indicates a vanished vegetated siliciclastic hinterland that may once have existed to the west or south from the present-day erosive edge of the Barrandian Devonian strata. Subvertical veins cutting the Koněprusy Limestone and filled with black solid bitumen and blackened calcite resulted from a subsequent but substantially later diagenetic event, which is a testament of aqueous and petroleum fluid migration through the succession during deeper burial. Microthermometric characteristics of the aqueous inclusions embedded in vein calcite indicate that the veins were precipitated by brines of low to moderate salinity (0.5–9.5 wt% NaCl equiv.) with temperatures in the range of 87–116°C. The bitumen in the veins is epi-impsonite (Rr = 0.70–1.90%), which is interpreted as degraded petroleum residuum that experienced thermal alteration at around 120°C. The AFT modeling combined with fluid inclusion microthermometry and wider geological considerations indicate that the veins originated during the Variscan orogeny, most probably upon deep burial of the Lower Paleozoic strata in Carboniferous time.

Discovery of a Mississippian Reef in Turkey: The Upper Viséan Microbial-Sponge-Bryozoan-Coral Bioherm From Kongul Yayla (Taurides, S Turkey)

For the first time, a Mississippian reef is described from Turkey. This microbial-sponge-bryozoan-coral bioherm has been discovered in the Central Taurides (South Turkey), at Kongul Yayla located between Hadim and Taşkent. The bioherm contains a rich and diversified fauna: sponges and rugose corals are of particular interest. The bioherm shows four main facies reflecting distinct growth stages from the base to the top: (1) the basal bioclastic beds, (2) the core facies formed of framestone comprising rugose corals, lithistid sponges, fistuliporid bryozoans and microbial boundstone, (3) the crest facies with large colonies of cerioid rugose corals and chaetetid sponges, and (4) the bioclastic facies containing reworked material from the bioherm in lateral and overlying positions to it. The entire bioherm is topped by siltstones with thin bioclastic horizons, often slumped. Siphonodendron pauciradiale and Lithostrotion maccoyanum are the guide taxa for the RC7\beta biozone and indicate an upper Asbian age for the bioherm. The Kongul Yayla bioherm resembles most the Cracoean reefs from northern England. It confirms the position of this buildup type along the platform margins and edges in the Palaeotethyan realm as seen in the British Isles, Belgium, southern France, southern Spain and North Africa. Facies and the coral fauna argue for a European affinity of the Anatolian terrane.

Evolution of pelagic swells from hardground analysis (Bathonian–Oxfordian, Eastern External Subbetic, southern Spain)

The Middle Bathonian to Middle Oxfordian interval in the Eastern External Subbetic (Betic Cordillera, SE Spain) is characterized by Ammonitico Rosso facies including various stratigraphic breaks. Five hardground-bounded units are recognized in relation to hiatuses in the ammonite record at the following stratigraphic boundaries: Hg1 (Lower–Middle Bathonian), Hg2 (Middle–Upper Bathonian), Hg3 (Lower–Middle Callovian), Hg4 (Middle–Upper Callovian), and Hg5 (Callovian–Oxfordian). Interesting features of these hardgrounds include their microfacies, ferruginous crusts and macro-oncoids, taphonomy of macroinvertebrates, trace fossils, neptunian dykes, and the hiatuses associated with each of them. The main hardgrounds (Hg1, Hg2, and Hg5) contain trace fossils of the Cruziana and Trypanites ichnofacies as well as abundant fossil macroinvertebrates with taphonomic features evidencing corrasion, early diagenesis, and reworking, indicating substrate evolution from softground to hardground. Neptunian dykes affected the trace fossils and ammonoid moulds, and their walls and the hardground surfaces were colonized by ferruginous microbial crusts. These features are characteristic of the External Subbetic pelagic swells, where the absence of sedimentation, sediment bypassing and erosion, and early diagenesis during relative sea-level falls produced hardgrounds. The neptunian dykes are indicative of tectonic activity in the areas of pelagic swells. Ferruginous crusts and macro-oncoids developed only on hardground surfaces and neptunian dykes walls prior to deposition of condensed bioclastic beds, which are interpreted as the first deposits after hardground development and are related to the onset of transgression. The varying ranges of the gaps as well as lateral facies changes are related to different local paleobathymetry controlled by the activity of listric faults.

Succession of rudistid lithosomes along the western coastal margin of the Iberian Basin (Coniacian, Castrojimeno Section, central Spain)

Rudistid lithosomes cropping out near Castrojimeno, at the northern margin of the Central System in north-central Spain, provide detailed information on their composition and structure, on their development and succession, and about their relationship with the Coniacian sequence stratigraphy framework of the Iberian Basin. Most rudist assemblages are oligospecific, with a dominant species, or monospecific. The radiolitids Biradiolites angulosus, Praeradiolites requieni, and Radiolites sauvagesi and the hippuritids Hippurites incisus and Vaccinites giganteus were identified. Radiolitids demonstrate wide intraspecific morphological variability. The following Riding’s structural categories of organic reefs are represented: segment reefs, spaced and close cluster reefs, and close cluster/frame reefs. Bioclastic beds of reworked rudist fragments occur below or in between the rudist reefs. The vertical succession of all five types of rudistid lithosomes distinguished evidences a shallowing-upward trend. Rudistid lithosomes developed on the coastal margin during the superposition of the highstand sea-level stage of third- and fourth-order depositional sequences.

Tsunami-related deposits in temperate carbonate ramps, Sorbas Basin, southern Spain

Tsunami-related deposits occur in Upper Miocene (uppermost Tortonian–lowermost Messinian) temperate carbonates in the Sorbas Basin, SE Spain. These carbonates exhibit two distinct depositional models. At the northern margin, small, locally steepened ramps developed on an irregular palaeotopography. These ramps displayed bryozoan accumulations at the toe of submarine cliffs changing laterally to coralline algal rudstones to floatstones. A gentle homoclinal ramp extended along the southern margin of the basin. Bivalve (brachiopod/bryozoan)-rich carbonates formed in the mid-ramp, whereas coralline algal-rudstones spread over the outer-ramp, changing basinwards to packstones with planktonic foraminifers. During the tsunami event large amounts of sediment were eroded from the carbonate ramps and redeposited. Two types of tsunami deposits are intercalated in outer-ramp sediments at both margins of the basin. In the steep outer-ramps of the northern margin, some folded layers are eroded and overlain by convex upward, stratified megahummocks. In the southern ramp, an abnormal thick shell-debris bed (TSB) occurs. Distinctive sedimentary features of these tsunamites and the inferred inflow and backflow effects were controlled by different palaeotopographic profiles. At the northern margin, inflow tsunami wave(s) struck the steep ramps, causing folding of underlying beds and excavating a large, irregular erosive surface. Backflow surges filled the inflow scours with the removed sediment, producing the megahummocky sets. At the southern margin, incoming tsunami surge(s) crossed the gentle ramp and eroded the sediments, especially in the inner-ramp settings. Backwash transported part of the previously removed sediments basinwards, depositing them as a thick bioclastic bed on the outer-ramp. The triggering mechanism of the tsunamis was probably related to seismic events recorded in the adjacent Tabernas Basin as several coeval seismites.

Geology and geochemistry of wall-rock alteration at the Carlin gold deposit, Nevada

The Carlin disseminated gold deposit occurs in an autochthonous sequence of Paleozoic sedimentary rocks exposed in a structural window in the Roberts Mountains thrust in north-central Nevada. The upper 175 m of the Silurian Roberts Mountains Formation hosts the majority of ore at Carlin and is characterized by laminated, fine-grained, calcareous and/or dolomitic argillaceous siltstone with local coarser grained siltstones and 50-cm-thick lenticular interbeds of sand- and granule-sized calcareous bioclastic debris or fossil hash. Detailed studies of drill core and exposures in the East pit of the Carlin mine show that alteration and mineralization are zoned away from crosscutting fault conduits and these more permeable bioclastic beds, indicating that these two features were major inflow zones for hydrothermal fluid.In unoxidized rocks, unaltered calcareous siltstone (1) containing quartz, dolomite, calcite, illite, K feldspar, and pyrite is progressively converted to assemblages of (2) quartz + dolomite + calcite + illite + pyrite, (3) quartz + dolomite + illite-K mica + pyrite, (4) quartz + illite-K mica + pyrite, and (5) quartz + kaolinite-dickite + pyrite adjacent to inflow zones where jasperoids are developed. Gold most consistently enriches the zone of calcite and dolomite removal (3 and 4 above), though it occurs in all zones, locally in high concentrations. This zoned alteration was accomplished by a CO 2 -rich acidic fluid. This acidic alteration enhanced the passage of fluids by extensive carbonate removal to form zones of higher permeability.Oxidation is wholly a supergene effect related to deep weathering, because the oxidation is superimposed on both mineralized and altered rocks with only minor effect on the major element chemistry; it has produced low-temperature goethitic Fe oxides rather than higher temperature hematite and is not spatially related to Au distribution at the mine or on a district scale.Because of extensive carbonate removal leading to local volume reduction through collapse and/or compaction, geochemical effects are examined using ratios to relatively immobile elements such as Al and Ti. Extensive depletion of Ca, Mg, aud CO 2 and introduction of Si, Au, and S have occurred. Potassium is depleted in the conversion of illite to dickite-kaolinite in proximal silicified inflow zones, and Fe enriches some pyritized rock. Carbonate removal and silicification are two separate processes, both of which are spatially associated with mineralization. Mineralized decarbonated rocks and barren footwall rocks commonly are not silicified, and intensely silicified proximal alteration zones are generally low grade.

The Goldenville–Halifax transition, Mahone Bay, Nova Scotia: relative sea-level rise in the Meguma source terrane

In the Meguma Group of Nova Scotia, the transition from sand-rich turbidites of the Goldenville Formation to overlying mud-rich turbidites of the Halifax Formation shows unusually rapid vertical facies variations. The Tancook member of the Goldenville Formation is divided into four laterally continuous units. Unit 1 consists of generally thinly bedded classical turbidites interpreted as lobe facies. Unit 2 comprises mottled (bioturbated) and laminated muds. Units 3 and 4 contain packets of thick amalgamated sandstone beds, alternating with more thinly bedded muddy sections. A unique bioclastic bed occurs near the top of unit 3. In unit 4, burrowed sandstone packets traced along strike (across paleocurrent) are not significantly incised into underlying muds. Laminated slates and siltstones of the overlying Mosher's Island member (Halifax Formation) are enriched in manganese and trace metals and contain diagenetic concretions of manganoan carbonate rimmed by metamorphic garnet. The overlying Cunard member comprises dark grey to black, strongly cleaved, carbon-rich slates, interbedded with thin, pyrite-rich, graded siltstones.Changes in the transition zone can be explained by a relative sea-level rise in the Gondwanaland source area of the Meguma Group. During deposition of the Tancook member, turbidite sand supply was progressively and intermittently interrupted as sand became trapped on the shelf. In unit 1, deposition occurred in a submarine-fan system; in higher units, deposition of sandstone packets was probably controlled by transgressions and regressions in the source area. Expanded shelves, briefly colonized by a shelly fauna, led to high organic productivity and eventual anoxia. During deposition of the Mosher's Island member, manganese was concentrated in reducing waters, delivered to the sea floor close to the oxic–anoxic boundary, and trapped as carbonate during early diagenesis. Expansion of fully anoxic conditions led to deposition of black, sulphide-rich shales of the Cunard member.

Conodont distributions, and ages of Silurian and Devonian limestones in the Palmers Oakey district, N.S.W.

Abstract Limestones in the Palmers Oakey District, N.S.W., range in age from Middle Silurian to Early Devonian. They occur as sheet‐like, bioclastic beds and/or isolated clasts in three facies: a volcaniclastic facies, a mudstone‐limestone facies, and a limestone‐mudstone breccia facies. Multielement conodont taxa, not previously reported from Australia, include Kockelella absidata, Kockelella stauros, Ozarkodina asymmetrica, Ozarkodina stygia, Ozarkodina transitans, Pseudooneotodus bicornis, and Pseudooneotodus tricornis.

Depositional Cycles of Lodgepole Formation (Mississippian) in Central Montana: ABSTRACT

Detailed stratigraphic and petrographic investigations of lower Madison Group outcrops reveal that the Lodgepole Formation is composed of 5 two-part depositional cycles. The stratigraphically lowest cycle includes the entire Paine Member; the remaining cycles comprise all but the lower part of the Woodhurst Member. Each cycle is characterized by a fine-grained lower unit and a coarser upper unit. The lower unit is dominated by horizontally laminated carbonate mudstones, pellet carbonate grainstones, and finely crystalline dolomites. These lithologies are interpreted to be the deposits of calm, nonturbulent lithotopes. The upper unit of each cycle is characterized by cross-laminated, medium- to coarse-grained, bioclastic and oolitic carbonate grainstones, interpreted to have been deposited in shallow, turbulent environments. From the lithology, sedimentary structures, lateral petrographic and stratigraphic continuity, and modern analogues, these oolitic and bioclastic beds are interpreted to be generally synchronous within individual outcrop belts in central Montana. Regionally, however, cycle-ca ping intervals are probably diachronous stratigraphic units. Facies interpretations of Lodgepole depositional cycles suggest that rocks of the fine-grained lower unit are deposits of deeper water transgressive phases of the Lodgepole sea; lithologies of the upper coarse-grained unit are accumulations of the shallower water regressive phases. End_of_Article - Last_Page 654------------