Control of inherited accreted lithospheric heterogeneity on the architecture and the low long-lived subsidence rate of intracratonic basins

Intracratonic basins tend to subside much longer than the timescale predicted by thermal relaxation of the lithosphere. Many hypotheses have been suggested to explain their longevity, yet few have been tested using quantitative thermo-mechanical numerical models, which capture the dynamic of the lithosphere. Lithospheric scale geodynamic modelling preserving the tectono-stratigraphic architecture of these basins is challenging because they display only few kilometres of subsidence over 1000 of km during time periods exceeding 250 Myr. Here we present simulations that are designed to examine the relative role of thermal anomaly, tectonics and heterogeneity of the lithosphere on the dynamics of intracratonic basins. Our results demonstrate that initial heterogeneity of accretionary continental lithosphere explains long-term subsidence and the arches-basins architecture of Saharan type intracratonic basins at first order. The simulations show that initially warm and heterogeneous lithospheres inherited from accretion are strong enough to resist local isostatic re-equilibration for very long period of time. Indeed, the lateral density variations store potential gravitational energy that is then slowly dissipated by differential erosion and slow vertical movements. For relatively well-accepted coefficient of erosion of 10-6 m2/s, the subsidence last longer than 250 Myr. Extensional tectonic forcing and thermal anomalies both results in an effective strength drop of the lithosphere, which allows a temporarily acceleration of local isostatic re-equilibration. Periodic changes in far field tectonic forcing from extension to compression complicate the tectono-stratigraphic architecture (intra-basin arches, sub-basins) introducing stratigraphic unconformities between different neighbouring basins such as the ones observed in North Africa.

Trilobite fauna (Wuliuan Stage, Miaolingian Series, Cambrian) of the lower Lakeview Limestone, Pend Oreille Lake, Idaho

The Lakeview Limestone is one of the westernmost Cambrian exposures in the northwestern United States and occurs on the western edge of the Montania paleotopographic high. These deposits occur between the deeper water deposits to the west and carbonate banks and intracratonic basins to the east and provide critical link between the regions. A re-investigation of the Cambrian trilobite faunas from the lower portion of the Lakeview Limestone, Pend Oreille Lake, Idaho, is undertaken due to the inadequate illustrations and descriptions provided by Resser (1938a). Resser's type specimens and additional material are figured and described. The trilobite assemblages represent the Ptychagnostus praecurrens Zone, Wuliuan Stage, Miaolingian Series and including two new taxa: Itagnostus idahoensis n. sp., and Utia debra n. sp. Because of the similarity between some species of Amecephalus from the Lakeview Limestone to specimens from the Chisholm Shale, Nevada, the type specimens of Amecephalus piochensis (Walcott, 1886) and Am. packi (Resser, 1935), Walcott's and Resser's type specimens are re-illustrate and their taxonomic problems are discussed. Utia curio Walcott, 1924 from the Spence Shale, Utah, have never been re-illustrated since Walcott (1925), these type specimens are also re-illustrated and compared to Utia debra n. sp.UUID: http://zoobank.org/5ef6ee5e-bddc-4395-becf-9bcea54c679c

Assessing the rift to sag evolution of Parnaiba Basin, NE Brazil, through U-Pb detrital zircon geochronology and provenance

<p><strong>Assessing the rift to sag evolution of Parnaíba Basin, NE Brazil, through U-Pb detrital zircon geochronology and provenance</strong></p><p>Rodrigo I. Cerri<sup>1</sup>; Lucas V. Warren<sup>1</sup>; Mario L. Assine<sup>1<br></sup><sup>1</sup> São Paulo State University (UNESP), Institute of Geosciences and Exact Sciences, Rio Claro, Brazil.</p><p>Nowadays one of the most prolific topics in the geological sciences is the origin of intracratonic basins. Despite many Paleozoic examples in which rift systems occur under these basins, there is no consensus about how these mechanical subsidence basins influenced the origin of continental-scale intracratonic basins. Due to its inherent complexity, the understanding of this problem only comes from integrated studies based in multi-proxy analysis, placing it on the frontier of modern science. In the northeast part of Brazil, the Late-Precambrian to Early-Cambrian Jaibaras Basin is interpreted as the precursor rift of the Parnaíba intracratonic Basin, following a simple model of mechanical-to-thermal subsidence evolution. In order to assess the provenance patterns and maximum depositional ages (MDA) between the rift and cratonic phases of these basins, we present a novel detrital zircon U-Pb ages of rocks from the Aprazível and Ipu formations. The main goals of this approach is to identify provenance changes (or similarities) between the last rift related sedimentary unit of the Jaibaras Basin and the first intracratonic related sedimentary unit form the Parnaíba Basin, thus allowing to test the rift-to-sag hypothesis. The MDA for the Aprazível Formation (ca. 499 ± 5 Ma, Furongian to Miaolingian) indicates a Late Cambrian age for the upper part of the Jaibaras Basin. The Ipu Formation records a MDA of ca. 528 ± 11 Ma (Terreneuvian to Series 2, Early Cambrian). However, due to its stratigraphic position relative to the lower Aprazível (499 ± 5 Ma) and upper Tianguá (Early Silurian, Llandovery) formations, the depositional age of this unit is probably younger (Late-Cambrian to Early-Ordovician). Thus, the successions deposited in the end of the rift and the beginning of the sag phase are clearly separated by a regional unconformity (10 to 30 Ma). We also identify the complete absence of Cambrian zircons followed by a significant increase in Paleoproterozoic ones in the Ipu Formation. Although these units were significantly sourced by Neoproterozoic terrains (especially Ediacaran), this modification indicates an interesting change in provenance between the rift to sag basins. The detrital zircon provenance, helped by a consistent paleocurrent analysis, reveal local source areas for the Aprazível Formation and a consistent distal sedimentary transport towards NW for the Ipu Formation. This suggests that the primary sources for the first cratonic unit of Parnaíba Basin were located at the orogenic areas related with the Neoproterozoic Brasiliano/Pan-African Orogeny at the south/southern of Borborema Province (e.g. Rio Preto, Riacho do Pontal and Rio Grande do Norte metamorphic belts). Unlike the alluvial-related Aprazível Formation, the Ipu Formation characterizes a huge fluvial system that flowed towards NW, probably following a homoclinal ramp-like tilted and opened to the paleomargins of Gondwana.</p>

Ascocerid cephalopods from the Hirnantian?–Llandovery stages of the southern Paraná Basin (Paraguay, South America): first record from high paleolatitudes

Ascocerid cephalopods are described for the first time from high paleolatitudes of Gondwana. Studied material was collected from the Hirnantian?–Llandovery strata of the Eusebio Ayala and Vargas Peña formations, Paraná Basin, southeastern Paraguay. The specimens are poorly preserved and were questionably assigned to the subfamily Probillingsitinae Flower, 1941, being undetermined at genus and species rank because diagnostic characters are not visible. A particular feature seen in our material is the presence of both parts of the ascocerid conch (the juvenile or cyrtocone and the mature or brevicone) joined together, which is a very rare condition in the known paleontological record. The specimens are interpreted as at a subadult stage of development because fully grown ascocerids would have lost the juvenile shell. A planktonic vertical migrant mode of life with a subvertical attitude is proposed for the juvenile, and a horizontal demersal nektonic mode for the adult form, as has been previously suggested. A subvertical orientation near the bottom is proposed for the subadult stage. We suggest that the immigration of ascocerids to southwestern Gondwana was possible through ocean currents that would carry the planktonic juveniles from low to high latitudes during the end-Ordovician postglacial transgression that flooded the intracratonic basins of the region.