Frasnian Strata Research Articles

Paleoecologic trends of Devonian Malvinokaffric fauna from the Paraná Basin as evidenced by trace fossils

The Malvinokaffric Realm is an endemic paleogeographic region that inhabited Gondwanan high latitude seas during Early-Middle Devonian times. The causes for the collapse of this fauna are still debated but seem to be related to a transgressive event during Middle Devonian in the Chaco and Paraná basins in Bolivia and Brazil, which probably generated warmer water currents into the seas bearing Malvinokaffric fauna. This study applies ichnology as a relative proxy to better understand the impact of the main paleobiological events in the collapse of the Malvinokaffric Realm. Eight Devonian ichnoguilds are recognized in this study (Asterosoma, Glossifungites, Macaronichnus, Phycosiphon, Planolites, Skolithos, Teichichnus, and Zoophycos), and their stratigraphic distribution was compared with macrofossil distribution in Paraná Basin. The Pragian to Middle Emsian strata represent the climax of the diversity of body and trace fossils, recording distinct invertebrate life habits; the Asterosoma and Skolithos ichnoguilds predominate in the southern and northwestern borders, respectively, the last representing proximal settings. The “first decline” in Malvinokaffric diversity is diagnosed during the Eifelian, after an extensive regressive event during the late Emsian characterized by the reduction in different life habits, mostly suspension- and detritus-feeding. Trace fossils represent the Planolites ichnoguild principally, and the Eifelian strata are virtually absent on the northwest edge close to the paleo-basin flanks. The Givetian interval is characterized by the most expressive flooding event (Káčak event) that connected several Gondwanic basins, allowing the presence of extra-Malvinokaffric taxa in the Paraná Basin. Trace fossils are represented chiefly by the Phycosiphon ichnoguild in the Eifelian–Givetian boundary in the southern edge, and Planolites, Asterosoma, and Skolithos ichnoguilds in the upper strata. Finally, the Frasnian strata were diagnosed only to the northwestern edge of the basin, presenting a very low macrofossil diversity (e.g., lingulids and Australocoelia), and a low-ichnodiversity progradational trend from Phycosiphon to Skolithos ichnoguild. The identified paleoenvironmental changes evidences that the Early Devonian shallow seas (Jaguariaíva Member) were impacted by a gradual increase in freshwater input towards the Middle Devonian.

Middle to Late Devonian distal braidplain deposition in the northeast sector of the Darling Basin Conjugate Fault System, near White Cliffs settlement, western New South Wales

Sandstone-rich, 6 km-thick, Eifelian–Frasnian strata of the northeast sector of the Darling Basin Conjugate Fault System, lie north of the Darling River Lineament, east of the Koonenberry Fault, and south of the Olepeloko Fault. Fossils are very rare in the study area: exceptions are in the Skolithos-rich beds of the Paroo National Park, and the early Eifelian fish-fragment horizon near Mt Jack Station. Four marker horizons are known southeast of the Paroo National Park: (i) the Mt Jack fish-fragment horizon; (ii) a Skolithos-rich belt at Momba Station; (iii) the mid-Givetian Keginni Conglomerate Member; and (iv) a ∼20 m-thick vein quartz clast-rich unit that crops out near the base of the Frasnian strata. Four braidplains are mapped: (i) White Cliffs (largest); (ii) Mt Jack; (iii) Bullocky Bore; and (iv) the distal part of the Keginni Conglomerate Member. Sediment derivation was from west of the Koonenberry Fault (White Cliffs Braidplain) from the northwest (Mt Jack Braidplain), from the north (Bullocky Bore Braidplain) and from the southeast (Keginni Conglomerate Braidplain). The braided streams/rivers depositing the sheet braidplains were very shallow, and sheet-flood deposition on the braidplains was widespread.

Note onCyphaspis Burmeister 1843 (Trilobita, Aulacopleuridae), with a review of several species mainly from the Devonian of Belgium and Germany

Specimens ofCyphaspisBurmeister are recorded from Eifelian and Frasnian strata in Belgium as well as from Eifelian strata in Germany, and are assigned to six taxa. Scant information has hitherto been available onCyphaspis from the Ardennes. From this regionCyphaspis unguloides belgica n. ssp.,Cyphaspis sp. B,Cyphaspisl sp. C, andCyphaspis sp. D are recorded. Newly collected topotypical material ofCyphaspis unguloides unguloides (Erben) from the Eifel is described which permits current assignment of this taxon toCyphaspis. The holotype ofCyphaspis hoepfneri hoepfneri (Kowalski) from the Eifel, which was previously only known from poor illustrations, is refigured. The diagnosis ofCyphaspis is emended to include several taxa that exhibit a relatively weakly vaulted glabella and occasionally shorter genal spines.HeliopygeHaas & Mensink (Acastidae), which is in need of revision, is briefly discussed.

NEW STYGINIDS FROM THE LATE DEVONIAN OF WESTERN AUSTRALIA— THE LAST CORYNEXOCHID TRILOBITES

Two new species of scutelluine styginid trilobites are described from Frasnian strata in the Virgin Hills Formation in the Canning Basin of Western Australia. They are placed in the genus Telopeltis n. gen., reflecting the fact that their final occurrence in the late Frasnian, up to the latest Frasnian Kellwasser extinction event, is the last known record of scutelluine trilobites. As such, it also represents the youngest record of the order Corynexochida. The two species, Telopeltis woodwardi n. sp. and Telopeltis microphthalmus n. sp., are unlike most other scutelluines in possessing extremely vaulted pygidia and showing trends to eye reduction. Such eye reduction is a common feature of late Frasnian trilobites. The characteristic morphological features of this small genus are indicative of evolution of this last scutelluine by paedomorphosis.

Carbonate platform evolution: from a bioconstructed platform margin to a sand‐shoal system (Devonian, Guilin, South China)

ABSTRACT The depositional organization and architecture of the middle–late Devonian Yangdi rimmed carbonate platform margin in the Guilin area of South China were related to oblique, extensional faulting in a strike‐slip setting. The platform margin shows two main stages of construction in the late Givetian to Frasnian, with a bioconstructed margin evolving into a sand‐shoal system. In the late Givetian, the platform margin was rimmed with microbial buildups composed mainly of cyanobacterial colonies (mostly Renalcis and Epiphyton). These grew upwards and produced an aggradational (locally slightly retrogradational) architecture with steep foreslope clinoforms. Three depositional sequences (S3–S5) are recognized in the upper Givetian strata, which are dominated by extensive microbialites. Metre‐scale depositional cyclicity occurs in most facies associations, except in the platform‐margin buildups and upper foreslope facies. In the latest Givetian (at the top of sequence S5), relative platform uplift (± subaerial exposure) and associated rapid basin subsidence (probably a block‐tilting effect) caused large‐scale platform collapse and slope erosion to give local scalloped embayments along the platform margin and the synchronous demise of microbial buildups. Subsequently, sand shoals and banks composed of ooids and peloids and, a little later, stromatoporoid buildups on the palaeohighs, developed along the platform margin, from which abundant loose sediment was transported downslope to form gravity‐flow deposits. Another strong tectonic episode caused further platform collapse in the early Frasnian (at the top of sequence S6), leading to large‐scale breccia release and the death of the stromatoporoid buildups. Siliceous facies (banded cherts and siliceous shales) were then deposited extensively in the basin centre as a result of the influx of hydrothermal fluids. The platform‐margin sand‐shoal/bank system, possibly with gullies on the slope, persisted into the latest Frasnian until the restoration of microbial buildups. Four sequences (S6–S9), characterized by abundant sand‐shoal deposits on the margin and gravity‐flow and hemipelagic deposits on the slope, are distinguished in the Frasnian strata. Smaller‐scale depositional cyclicity is evident in all facies associations across the platform–slope–basin transect. The distinctive depositional architecture and evolution of this Yangdi Platform are interpreted as having been controlled mainly by regional tectonics with contributions from eustasy, environmental factors, oceanographic setting, biotic and sedimentary fabrics.

Carbonate sedimentation in a starved pull‐apart basin, Middle to Late Devonian, southern Guilin, South China

ABSTRACT Geological mapping and sedimentological investigations in the Guilin region, South China, have revealed a spindle‐ to rhomb‐shaped basin filled with Devonian shallow‐ to deep‐water carbonates. This Yangshuo Basin is interpreted as a pull‐apart basin created through secondary, synthetic strike‐slip faulting induced by major NNE–SSW‐trending, sinistral strike‐slip fault zones. These fault zones were initially reactivated along intracontinental basement faults in the course of northward migration of the South China continent. The nearly N–S‐trending margins of the Yangshuo Basin, approximately coinciding with the strike of regional fault zones, were related to the master strike‐slip faults; the NW–SE‐trending margins were related to parallel, oblique‐slip extensional faults. Nine depositional sequences recognized in Givetian through Frasnian strata can be grouped into three sequence sets (Sequences 1–2, 3–5 and 6–9), reflecting three major phases of basin evolution.During basin nucleation, most basin margins were dominated by stromatoporoid biostromes and bioherms, upon a low‐gradient shelf. Only at the steep, fault‐controlled, eastern margin were thick stromatoporoid reefs developed. The subsequent progressive offset and pull‐apart of the master strike‐slip faults during the late Givetian intensified the differential subsidence and produced a spindle‐shaped basin. The accelerated subsidence of the basin centre led to sediment starvation, reduced current circulation and increased environmental stress, leading to the extensive development of microbial buildups on platform margins and laminites in the basin centre. Stromatoporoid reefs only survived along the windward, eastern margin for a short time. The architectures of the basin margins varied from aggradation (or slightly backstepping) in windward positions (eastern and northern margins) to moderate progradation in leeward positions. A relay ramp was present in the north‐west corner between the northern oblique fault zone and the proximal part of the western master fault. In the latest Givetian (corresponding to the top of Sequence 5), a sudden subsidence of the basin induced by further offset of the strike‐slip faults was accompanied by the rapid uplift of surrounding carbonate platforms, causing considerable platform‐margin collapse, slope erosion, basin deepening and the demise of the microbialites. Afterwards, stromatoporoid reefs were only locally restored on topographic highs along the windward margin. However, a subsequent, more intense basin subsidence in the early Frasnian (top of Sequence 6), which was accompanied by a further sharp uplift of platforms, caused more profound slope erosion and platform backstepping. Poor circulation and oxygen‐depleted waters in the now much deeper basin centre led to the deposition of chert, with silica supplied by hydrothermal fluids through deep‐seated faults. Two ‘subdeeps’ were diagonally arranged in the distal parts of the master faults, and the relay ramp was destroyed. At this time, all basin margins except the western one evolved into erosional types with gullies through which granular platform sediments were transported by gravity flows to the basin. This situation persisted into the latest Frasnian.This case history shows that the carbonate platform architecture and evolution in a pull‐apart basin were not only strongly controlled by the tectonic activity, but also influenced by the oceanographic setting (i.e. windward vs. leeward) and environmental factors.

A late Devonian (Frasnian) coral-bafflestone reef at Houshan in Guilin, South China

Givetian to early Carboniferous sediments of South China are characterized by carbonates. Middle and Late Devonian strata are best developed in the Guilin area. Reefs and organic shoals are recorded by various lithofacies types indicating the existence of an extended carbonate platform and a change of the composition of reef communities in time. Starting in the late Devonian, stromatoporoids and corals were replaced by algae that subsequently played an important role together with stromatoporoids, receptaculitids and fasciculate rugose corals in reef communities. In Houshan, 5 km west of Guilin, a coral-bafflestone reef occurs in the Frasnian strata, situated near an offshore algal-stromatoporoid reef. The coral reef was formed in a back-reef area adjacent to the inner platform margin. The coral-bafflestone reef is unique among the late Devonian reefs of South China with regard to the biotic composition. The reef is composed of fasciculate colonies ofSmithiphyllum guilinense n. sp. embedded within in packstones and wackestones. The height of colonies reaches 1 m. The community is low-diverse. The species ofSmithiphyllum occurring in the Frasnian reef complexes of Guilin exhibit a distinct facies control:Smithiphyllum guilinense occurs in or near to margin facies and formed bafflestone, constituting a coral reef whereasSmithiphyllum occidentaleSorauf, 1972 andSmithiphyllum sp.—characterized by small colonies with thin corallites—are restricted to the back-reef and marginal slope facies. The bush-like coral colonies baffled sediments. Algae and stromatoporoids (mainlyStachyodes) are other reef biota. Reef-dwelling organisms are dominated by brachiopods.