Nicolas Olivier

Microbialite morphology, structure and growth: a model of the Upper Jurassic reefs of the Chay Peninsula (Western France)

Abstract During the Early Kimmeridgian, the northern margin of the Aquitaine Basin (Western France) is characterised by a significant development of coral reefs. The reef formation of the Chay Peninsula comprises two main reefal units, in which the microbial structures can contribute up to 70% of framework. The microbial crusts, which played an important role in the stabilisation and growth of the reef body, show the characteristic clotted aspect of thrombolitic microbialites. Corals are the main skeletal components of the build-ups. The bioconstructions of the Chay area are thus classified as coral-thrombolite reefs. Four main morpho-structural types of microbial crusts are distinguished: (1) pseudostalactitic microbialites on the roof of intra-reef palaeocaves; (2) mamillated microbialites, found either on the undersides or on the flanks of the bioherms; (3) reticular microbialites in marginal parts of the reefs and between adjacent bioconstructed units; and (4) interstitial microbialites in voids of bioclastic deposits. Thrombolitic crusts developed on various substrates such as corals, bivalves, or bioclasts. The thrombolites formed a dense, clotted and/or micropeloidal microbial framework, in which macro- and micro-encrusters also occur. Variations in accumulation rate strongly influenced the reef morphology, in particular its relief above the sediment surface. The coalescence of the coral-microbialite patches created numerous intra-reef cavities of metre-scale dimensions. The direction of microbial growth, which defined the macroscopic microbialite forms, strongly depended on the position within the reef framework but was also controlled by water energy, accumulation rate and light availability.

Smithian ammonoid faunas from Utah: implications for Early Triassic biostratigraphy, correlation and basinal paleogeography

Intensive sampling of the lower portion of the Thaynes and Moenkopi Groups (Lower Triassic) at separate localities within the Confusion Range, Pahvant Range, Mineral Mountains, Star Range, Kanarraville, Cedar City, Torrey and San Rafael Swell areas (mainly central and southern Utah, USA) leads to the recognition of a new key regional Smithian ammonoid succession. The new biostratigraphical sequence, which is more precise than the long-recognized Meekocerasgracilitatis and Anasibiriteskingianus Zones, comprises twelve subdivisions, thus resulting in a sequence with much higher resolution that can be correlated not only with other western USA sites, but also with major worldwide localities as well. Middle and late Smithian faunas contain many taxa with wide geographic distribution, thus enabling long-distance correlation with faunal successions from other regions (e.g., British Columbia, Canadian Arctic, South China, Spiti and Oman). New assemblages from the lowermost beds are the least diversified and poorest preserved; they represent the earliest early/middle Smithian ammonoid faunas reported from the western North American basin. They highlight (a) the sudden Smithian advancement of the marine transgression within this epicontinental sea, (b) that this event is diachronous, and (c) that the paleotopography of the basin most likely was highly irregular. The newly obtained ammonoid succession also allows us to date and follow the transgression from the northern and central part of the basin to the southwesternmost and southeasternmost parts, which were reached during the late Smithian (Anasibiriteskingianus beds). In addition, we briefly discuss the now-limited previous regional biozonation in the light of these new results. One new genus (Minersvillites) and nine new species (Kashmirites utahensis, Kashmirites confusionensis, Kashmirites stepheni, ?Xiaoqiaoceras americanum, Minersvillites farai, Inyoites beaverensis, Meekoceras olivieri, Meekoceras millardense, Vercherites undulatus) are also described.

Lower crustal flow kept Archean continental flood basalts at sea level

Large basaltic provinces as much as 15 km thick are common in Archean cratons. Many of these flood basalts erupted through continental crust but remained at sea level. Although common in the Archean record, subaqueous continental flood basalts (CFBs) are rare to absent in the post-Archean. Here we show that gravity-driven lower crustal flow may have contributed to maintaining Archean CFBs close to sea level. Our numerical experiments reveal that the characteristic time to remove the thickness anomaly associated with a CFB decreases with increasing Moho temperature (T(M)), from 500 m.y. for T(M) approximate to 320 degrees C to 1 m.y. for T(M) approximate to 900 degrees C. This strong dependency offers the opportunity to assess, from the subsidence history of CFBs, whether continental geotherms were significantly hotter in the Archean. In particular, we show that the subsidence history of the ca. 2.7 Ga upper Fortescue Group in the East Pilbara Craton, Western Australia, requires Moho temperatures >>700 degrees C. Applied to eight other unambiguous subaqueous Archean CFBs, our results indicate Moho temperatures >>650 degrees C at the time of eruption. We suggest that the decrease in the relative abundance of subaqueous CFBs over Earths history could reflect the secular cooling of the continental lithosphere due to the decrease in radiogenic heat production.

Unexpected Early Triassic marine ecosystem and the rise of the Modern evolutionary fauna.

A new marine fossil assemblage from the Early Triassic shows unexpected phyletic diversity and functional complexity. In the wake of the end-Permian mass extinction, the Early Triassic (~251.9 to 247 million years ago) is portrayed as an environmentally unstable interval characterized by several biotic crises and heavily depauperate marine benthic ecosystems. We describe a new fossil assemblage—the Paris Biota—from the earliest Spathian (middle Olenekian, ~250.6 million years ago) of the Bear Lake area, southeastern Idaho, USA. This highly diversified assemblage documents a remarkably complex marine ecosystem including at least seven phyla and 20 distinct metazoan orders, along with algae. Most unexpectedly, it combines early Paleozoic and middle Mesozoic taxa previously unknown from the Triassic strata, among which are primitive Cambrian-Ordovician leptomitid sponges (a 200–million year Lazarus taxon) and gladius-bearing coleoid cephalopods, a poorly documented group before the Jurassic (~50 million years after the Early Triassic). Additionally, the crinoid and ophiuroid specimens show derived anatomical characters that were thought to have evolved much later. Unlike previous works that suggested a sluggish postcrisis recovery and a low diversity for the Early Triassic benthic organisms, the unexpected composition of this exceptional assemblage points toward an early and rapid post-Permian diversification for these clades. Overall, it illustrates a phylogenetically diverse, functionally complex, and trophically multileveled marine ecosystem, from primary producers up to top predators and potential scavengers. Hence, the Paris Biota highlights the key evolutionary position of Early Triassic fossil ecosystems in the transition from the Paleozoic to the Modern marine evolutionary fauna at the dawn of the Mesozoic era.

Smithian shoreline migrations and depositional settings in Timpoweap Canyon (Early Triassic, Utah, USA)

In Timpoweap Canyon near Hurricane (Utah, USA), spectacular outcrop conditions of Early Triassic rocks document the geometric relationships between a massive Smithian fenestral-microbial unit and underlying, lateral and overlying sedimentary units. This allows us to reconstruct the evolution of depositional environments and high-frequency relative sea-level fluctuations in the studied area. Depositional environments evolved from a coastal plain with continental deposits to peritidal settings with fenestral-microbial limestones, which are overlain by intertidal to shallow subtidal marine bioclastic limestones. This transgressive trend of a large-scale depositional sequencemarks a long-term sea-level rise that is identified worldwide after the Permian-Triassic boundary. The fenestral-microbial sediments were deposited at the transition between continental settings (with terrigenous deposits) and shallow subtidal marine environments (with bioturbated and bioclastic limestones). Such a lateral zonation questions the interpretation of microbial deposits as anachronistic and disaster facies in the western USA basin. The depositional setting may have triggered the distribution of microbial deposits and contemporaneous marine biota. The fenestral-microbial unit is truncated by an erosional surface reflecting a drop in relative sea level at the scale of a medium depositional sequence. The local inherited topography allowed the recording of small-scale sequences characterized by clinoforms and short-distance lateral facies changes. Stratal stacking pattern and surface geometries allow the reconstruction of relative sea-level fluctuations and tracking of shoreline migrations. The stacking pattern of these small-scale sequences and the amplitude of corresponding high-frequency sea-level fluctuations are consistent with climatic control. Large-andmedium-scale sequences suggest a regional tectonic control.

A deep subaqueous fan depositional model for the Palaeoarchaean (3.46 Ga) Marble Bar Cherts, Warrawoona Group, Western Australia

The 3.46 Ga Marble Bar Chert Member of the East Pilbara Craton, Western Australia, is one of the earliest and best-preserved sedimentary successions on Earth. Here, we interpret the finely laminated thin-bedded cherts, mixed conglomeratic beds, chert breccia beds and chert folded beds of the Marble Bar Chert Member as the product of low-density turbidity currents, high-density turbidity currents, mass transport complexes and slumps, respectively. Integrated into a channel-levee depositional model, the Marble Bar Chert Member constitutes the oldest documented deep-sea fan on Earth, with thin-bedded cherts, breccia beds and slumps composing the outer levee facies tracts, and scours and conglomeratic beds representing the channel systems.

Superstesaster promissor gen. et sp. nov., a new starfish (Echinodermata, Asteroidea) from the Early Triassic of Utah, USA, filling a major gap in the phylogeny of asteroids.

We describe Superstesaster promissor gen. et sp. nov., a starfish from the Smithian (Early Triassic) of Utah (USA) that fills a major gap in the fossil record of the Asteroidea. The post-Palaeozoic crown group Asteroidea are distinct from any of the diverse Palaeozoic forms. However, current understanding of the Palaeozoic–Mesozoic transition is blurred by a large gap in the fossil record between the Early Permian and the Middle Triassic. Building on the newly described taxon, a phylogenetic analysis investigates the relationships between Palaeozoic and Mesozoic Asteroidea. Including 30 species and 70 morphological characters, it is the most comprehensive phylogeny produced for fossil starfishes so far. Relationships among Palaeozoic forms remain poorly resolved, but their position in the tree is grossly consistent with stratigraphy. The tree topology implies the appearance of a wide range of morphologies during the Ordovician, a bottleneck during the end-Devonian events, and a second diversification during the Carboniferous, before a diversity decline at the end of the Permian. Superstesaster promissor nests above Palaeozoic taxa and appears as the sister group to the post-Palaeozoic Asteroidea. It represents the first record of a member of the stem group in the Mesozoic, and it likely reflects the ancestral morphology of the crown group. S. promissor shares with the crown group typical ambulacral and adambulacral plate shape and articulation. Phylogenetic relationships within the crown group remain poorly resolved, although three clades are consistent with molecular and morphological phylogenies available for extant forms: Valvatacea (Comptoniaster, Pentasteria, Advenaster, Noviaster), Forcipulatacea (Germanasterias, Argoviaster) and Velatida (Tropidaster, Protremaster). A Triassic and Jurassic radiation of the crown group is confirmed. Most Triassic and Jurassic forms do not share all synapomorphies with extant clades and usually represent separate clades or stem members of modern clades rather than true members of modern families. http://zoobank.org/urn:lsid:zoobank.org:pub:8CAC421A-DC0C-4EEB-B872-C7505FA8C2EA

Coral- and oyster-microbialite patch reefs in the aftermath of the Triassic–Jurassic biotic crisis (Sinemurian, Southeast France)

The end of the Triassic and the Early Jurassic are intervals characterised by profound biotic and environmental changes, accompanied by dramatic decreases in marine fauna diversity. Corals were strongly affected and assemblages underwent a severe reduction; compared with those of the Upper Triassic, the Early Jurassic is traditionally defined as holding a “reef gap”. A Sinemurian coral-microbialites patch reef, located in southern France in the Hérault department (Le Perthus locality), is here described. This bioconstruction developed in a shallow mixed siliciclastic-carbonate inner ramp setting. The reef volume is composed of up to 70% of an intercoral facies mostly microbialites, with subordinated sediments (approximately 20–30% of the intercoral facies). Therefore, the patch reef can be defined as a coral-microbialite bioconstruction, in which microbialites were the main framebuilders. The coral assemblage has low diversity and is dominated by massive to branching colonies of Chondrocoenia clavellata. This highlights the reef diversity after the T/J boundary crisis. The Le Perthus patch reef could have acted as an edge for the dominant currents and probably induced reductions in hydrodynamic energy and sedimentation on one of its sides. Consequently, it could have triggered the growth of small lateral bioconstructions, composed of oysters and microbialites, uniquely on one of its sides. The evolution of the facies shows that the Le Perthus patch reef grew in a shallowing-upward setting accompanied by an increase in siliciclastic inputs. The rate of bioerosion and the faunal assemblage suggest that the bioconstructions could have been developed in a mesotrophic environment.

Late Smithian microbial deposits and their lateral marine fossiliferous limestones (Early Triassic, Hurricane Cliffs, Utah, USA)

Recurrent microbialite proliferations during the Early Triassic are usually explained by ecological relaxation and abnormal oceanic conditions. Most Early Triassic microbialites are described as single or multiple lithological units without detailed ecological information about lateral and coeval fossiliferous deposits. Exposed rocks along Workman Wash in the Hurricane Cliffs (southwestern Utah, USA) provide an opportunity to reconstruct the spatial relationships of late Smithian microbialites with adjacent and contemporaneous fossiliferous sediments. Microbialites deposited in an intertidal to subtidal interior platform are intercalated between inner tidal flat dolosiltstones and subtidal bioturbated fossiliferous limestones. Facies variations along these fossiliferous deposits and microbialites can be traced laterally over a few hundreds of meters. Preserved organisms reflect a moderately diversified assemblage, contemporaneous to the microbialite formation. The presence of such a fauna, including some stenohaline organisms (echinoderms), indicates that the development of these late Smithian microbial deposits occurred in normal-marine waters as a simple facies belt subject to relative sea-level changes. Based on this case study, the proliferation of microbialites cannot be considered as direct evidence for presumed harsh environmental conditions.

Wet tropical climate in SE Tibet during the Late Eocene

Cenozoic climate cooling at the advent of the Eocene-Oligocene transition (EOT), ~33.7 Ma ago, was stamped in the ocean by a series of climatic events albeit the impact of this global climatic transition on terrestrial environments is still fragmentary. Yet archival constraints on Late Eocene atmospheric circulation are scarce in (tropical) monsoonal Asia, and the paucity of terrestrial records hampers a meaningful comparison of the long-term climatic trends between oceanic and continental realms. Here we report new sedimentological data from the Jianchuan basin (SE Tibet) arguing for wetter climatic conditions in monsoonal Asia at ~35.5 Ma almost coevally to the aridification recognized northwards in the Xining basin. We show that the occurrence of flash-flood events in semi-arid to sub-humid palustrine-sublacustrine settings preceded the development of coal-bearing deposits in swampy-like environments, thus paving the way to a more humid climate in SE Tibet ahead from the EOT. We suggest that this moisture redistribution possibly reflects more northern and intensified ITCZ-induced tropical rainfall in monsoonal Asia around 35.5 Ma, in accordance with recent sea-surface temperature reconstructions from equatorial oceanic records. Our findings thus highlight an important period of climatic upheaval in terrestrial Asian environments ~2–4 millions years prior to the EOT.