Thilo Bechstädt

Anisian (middle triassic) buildups of the Northern Dolomites (Italy): The recovery of reef communities after the permian/triassic crisis

SummaryAfter the end-Permian crisis and a global ‘reef gap’ in the early Triassic, reefs appeared again during the early Middle Triassic. Records of Anisian reefs are rare in the Tethys as well as in non-Tethyan regions. Most Anisian reefs are known from the western part of the Tethys but there are only very few studies focused on biota, facies types and the paleogeographical situation of these reefs. From the eastern part of the Tethys, Anisian reefs, reefal buildups or potential reef-building organisms have been reported from different regions of southern China. Most of the Anisian reefs known from western and central Europe as well as from southern China seem to be of middle and late Pelsonian age.The study area is situated in the northern Dolomites (South Tyrol, Italy) southeast of Bruneck (Brunico). It comprises the area between Olang (Valdaora) and Prags (Braies). The study is based on detailed investigations of the regional geology, stratigraphy and lithofacies (R. Zühlke, T. Bechstädt) as well as on a comprehensive inventory of Anisian reef organisms (B. Senowbari-Daryan, E. Flügel). These data are used in the discussion of the controls on the recovery of reefs during the early Middle Triassic.Most late Anisian reef carbonates studied are represented by allochthonous talus reef blocks of cubicmeter size. Small biostromal autochthonous mounds are extremely rare (Piz da Peres). The reef mounds as well as most of the reef blocks occur within the middle to late Pelsonian Recoaro Formation. They were formed on the middle reaches of carbonate ramps in subtidal depths, slightly above the storm wave base with only moderate water energy. Most lithotypes observed in the reef blocks correspond to sponge and/or algal bafflestones. Low-growing sessile organisms (Olangocoelia (sponge, alga?), sphinctozoan sponges, bryozoans, soleno-poracean algae, corals) and encrusting epibionts (sponges, porostromate algae, cyanophycean crusts, foraminifera, worms, microproblematica) created low cm-sized biogenic structures (bioconstructions) which baffled and bound sediment. Organic framework was only of minor importance; it is restricted to theOlangocoelia lithotype. Framework porosity was small in these reef mounds. Submarine carbonate cements, therefore, are only of minor importance s compared with Permian or Ladinian reefs. The relatively high number of lithotypes encountered in the reef blocks indicates a high biofacies diversity.Regarding the relative frequency, the diverse biota consist in descending order ofOlangocoelia, sponges (sphinctozoans, inozoans, siliceous sponges), bryozoans, porostromate algae and worm tubes. The sphinctozoans are characterized by small, mostly incrusting forms. The numerical diversity (species richness) is low compared with late Permian or Ladinian and late Triassic sphinctozoan faunas occurring within reefs.Following the sponges, monospecific bryozoans (Reptonoditrypa cauticaSchäfer & Fois) are the most common organisms in the reef limestones. Porostromate algae were restricted to areas within the bioconstructions not inhabited by sponges. The low-diverse corals had no importance in the construction of an organic framework.Surprisingly, microbial crusts are rare or even lacking in the investigated Anisian bioconstructions. This is in contrast to late Permian and Ladinian as well as Carnian reefs which are characterized by the abundance of specific organic crusts. The same comes true for‘Tubiphytes’ which is a common constituent in Permian, Ladinian and Carnian reef carbonates but is very rare in the Anisian of the Olang Dolomites. Instead of‘Tubiphytes’ different kinds of worm tubes (spirorbid tubes, Mg-calcitic tubes and agglutinated tubes) were of importance as epifaunal elements. Macrobial encrustations consisting of characteristic successions of sponges, bryozoans, algae, worm tubes and microproblematica seem to be of greater quantitative importance than in Ladinian reefs.Destruction of organic skeletons (predominantly of bryozoans) by macroborers (cirripedia?) is a common feature.The Anisian reef organisms are distinctly different from late Permian and from most Ladinian reef-builders. No Permian Lazarus taxa have been found.New taxa: Sphinctozoan sponges—Celyphia? minima n.sp.,Thaumastocoelia dolomitica n. sp.,Deningeria tenuireticulata n. sp.,Deningeria crassireticulata n. sp.,Anisothalamia minima n.g. n.sp., Inozoan sponges-Meandrostia triassica n.sp. Microproblematica-Anisocellula fecunda n.g. n.sp., Porostromate alga-Brandneria dolomitica n.g. n.sp.Most of our data are in agreement with the model described byFois & Gaetani (1984) for the recovery of reef-building communities during the Ansian but the biotic diversity seems to be considerably higher than previously assumed.Anisian deposition and the formation of the reef mounds within the Pelsonian Recoaro Formation of the Dolomites were controlled by the combined effects of synsedimentary tectonics and eustatic changes in sea-level. During several time intervals, especially the early Anisian (northern and western Dolomites: tectonic uplift), the early Pelsonian (eastern Dolomites: drowning) and the late Illyrian (wide parts of the Dolomites: uplift and drowning), the sedimentation was predominantly controlled by regionally different tectonic subsidence rates. The amount of terrigenous clastic input associated with synsedimentary tectonics (tectonic uplift of hinterlands) had a major influence on carbonate deposition and reef development. The re-appearance of reef environments in the Olang Dolomites was controlled by a combination of regional and global factors (paleogeographic situation: development of carbonate ramps; decreasing subsidence of horst blocks; reduced terrigenous input; moderate rise in sea-level).

Sedimentary patterns across the Lower–Middle Cambrian transition in the Esla nappe (Cantabrian Mountains, northern Spain)

In the carbonate platforms of the western Gondwana margin, the extinction recorded at the Lower–Middle Cambrian boundary is accompanied by a profound change in the style of carbonate deposition. The Lancara Formation of the Esla nappe (Cantabrian Mountains, northern Spain) contains a distinct sedimentary turnover due to a combination of tectonism, eustatic fluctuations, and immigration and colonization of new benthic communities, such as the youngest archaeocyathan assemblage of the entire Iberian Peninsula. During latest Early Cambrian times, a regressive trend is recorded in the Lancara Formation. This regression was recorded on a peritidal-dominant, homoclinal ramp that is topped by a tectonically induced discontinuity (D1). The latter surface marks the beginning of a last prograding, regressive tendency recorded on an intra-shelf ramp with ooidal/bioclastic shoals protecting archaeocyathan-microbial patch reefs. The overlying discontinuity (D2) corresponds to a major erosive unconformity, which coincides with the Lower–Middle Cambrian boundary in the Cantabrian Mountains. The subsequent, long-term, earliest Middle Cambrian rise in relative sea-level allowed deposition of low-relief, bioclastic shoals bearing a diverse and cosmopolitan assemblage of benthic fauna. Finally, the previous evolution is bounded by a third discontinuity (D3), which marks the beginning of a rhythmic sedimentation indicative of a major phase of tectonic breakdown and drowning of platforms recognised throughout southwestern Europe. Two associations of calcimicrobes occur in the latest Early Cambrian regressive trend of the Lancara Formation: (i) Proaulopora and Subtiflora are identified in peritidal, high-energy settings, lacking self-supported structures, whereas (ii) intergrowths of Epiphyton, Renalcis and Girvanella encrusted branching colonies and solitary archaeocyaths in protected (back-shoal) patch reefs. The latest Early Cambrian regression is correlated in southwestern Europe in both siliciclastic (Iberian Chains and Ossa–Morena) and carbonate-dominant platforms (Cantabrian Mountains, Montagne Noire and Sardinia). Its tops are recognised as diachronous unconformities ranging in age from early Bilbilian to the Bilbilian–Leonian or Lower–Middle Cambrian boundary.

Hydrothermal dolomites in SW Sardinia (Italy) and Cantabria (NW Spain): evidence for late- to post-Variscan widespread fluid-flow events

Abstract Epigenetic replacive and saddle dolomitisations, several hundred meters thick, affected Cambrian to Ordovician limestones across large areas (several hundred kilometers) of the SW Sardinian (Italy) mining district, as well as Paleozoic carbonates of the Cantabrian region (Spain). Prior to dolomitisation, the Cambrian limestones of Sardinia underwent ductile deformation and greenschist facies metamorphism. In Cantabria the dolomitisation affected mostly the already folded limestones of the Barcaliente and Valdeteja Fms of Carboniferous age. In both areas it can be assumed that late-Variscan hydrothermal events, which coincided with extensional tectonics, set brine circulation into motion. Dolomitisation may have occurred within circulation cells, which were driven by high heat flow. It is quite likely, that the dolomitisations are indicative of a widespread fluid-flow event that affected most of SW-Europe.

Late Cryogenian (Neoproterozoic) glacial and post-glacial successions at the southern margin of the Congo Craton, northern Namibia: facies, palaeogeography and hydrocarbon perspective

Abstract The geological paradox of at least two Neoproterozoic glacial intervals at tropical latitudes intercalated within carbonates remains an unsolved puzzle. Several conceptual models have been proposed to explain these apparent rapid swings between climatic extremes and the associated isotopic changes in sea-water chemistry. In Oman, post-glacial transgressive sedimentary successions represent important hydrocarbon source rocks. Source rock characteristics of Neoproterozoic post-glacial successions in other parts of the world (even if not directly correlatable) are, therefore, of special economic interest. This paper concentrates on the Ghaub Formation diamictite interval in northern Namibia and the major environmental change in the aftermath of the assumed glaciation. The relationship of the post-glacial sediments with the underlying different types of cap carbonate and diamictite successions is discussed, and a model of the succession of events is presented. The palaeotopography, caused mostly by ongoing tectonic activity including uplift on the scale of thousands of metres, strongly influenced the petroleum system created and played an important role for the hydrocarbon prospectivity of this post-glacial succession. Tectonic activity on the shelf of the southern margin of the Congo Craton was repeated, and different sub-basins were created before, during and after the Ghaub glaciation. The newly formed relief was flooded, and the different sub-basins were affected by restricted circulation for quite some time. This general scenario bears many similarities to the late Ordovician–early Silurian petroleum system, also formed during post-glacial sea-level rise.

Dolomitisation and brecciation along fault zones in the Cantabrian mountains

Abstract Dolomitisation, brecciation, and dedolomitisation occurring in fault zones in the Variscan fold and thrust belt in the Cantabrian Zone (Northern Spain) in three different tectonic units (Ponga, Picos de Europa and Esla) have been investigated and compared. Cementation patterns are similar in all units: dolomite cements always predate calcite cements in brecciated limestone (Barcaliente Fm.). Moreover, carbon and oxygen isotopic composition trends are comparable. However, the precipitating fluids are distinct in each fault zone, as evidenced by microthermometry and Raman spectroscopy. The evolution of the fault zones is illustrated by the precipitation of cements from different types of circulating fluids in several pulses.

Fossillagerstätten im mitteltriadischen Plattenkalk der Gailtaler Alpen (Kärnten, Österreich)

Fossillagerstatten von Ammoniten und Daonellen werden mikrofaziell und mikrofaunistisch beschrieben. Sie befinden sich an der Basis mitteltriadischer Plattenkalke der Gailtaler Alpen. Die Ablagerung erfolgte in etwas tieferem Wasser unter Gyttjaverhaltnissen. Im Bereich der Fossilvorkommen, aber auch schon im unterlagernden „Zwischendolomit“, treten dunne tuffitische Lagen auf. Diese sind, wie die Fossillagerstatte selbst, in die Trinodosuszone („Unteres Illyr“) zu stellen.

Impact of rift dynamics on palaeoenvironmental conditions and hydrocarbon system development (northern Upper Rhine Graben, SW Germany)

The Upper Rhine Graben (URG), a classical hydrocarbon (HC) province, is part of the European Cenozoic Rift System. The rift graben development has led to a complex basin fill of terrestrial and marine Cenozoic deposits, providing several HC source and reservoir units. The aim of this study is the multidisciplinary analysis of the palaeoenvironmental conditions and source-rock development within the transgressive marine intervals, and the palaeothermal history of the graben system to improve the understanding of the HC system development. Palaeoenvironmental conditions are strongly influenced by rift-related tectonic activity. Transgressive marine intervals in times of major subsidence show high terrestrial input from the graben shoulders, leading to mainly terrigenous gas-prone kerogen, while transgressive marine intervals during weak tectonic activity are dominated by marine–brackish palaeoenvironments and oil-prone kerogen. This differs clearly from the previously suggested HC potential of these intervals. Thermal maturation analysis shows nearly constant maturation with depth, which is atypical for burial-controlled maturation. It indicates significant secondary thermal overprinting related to long-lasting very hot hydrothermal fluid systems, concentrated along fault zones. Therefore the development of the depositional setting, kerogen composition, thermal maturation and the HC potential is directly linked to the dynamics of the rift system development. Supplementary material: detailed results of all three wells are available at https://doi.org/10.6084/m9.figshare.c.4183106