First microbial-archaeocyathan boundstone record from early Cambrian erratic cobbles in glacial diamictite deposits of Namibia (Dwyka Group, Carboniferous)

This study records the first occurrence of a lower Cambrian archaeocyathan fauna in limestone cobbles from Carboniferous glacigenic deposits of the Dwyka Group of the Aranos Basin in Namibia. Petrographic and microfacies analysis of 17 large, middle and standard thin sections reveals two distinctive limestone cobble facies: (a) bioclastic wackestone-packstone (hyoliths, trilobites, chancelloriids, crinoids and archaeocyaths); and (b) microbial-archaeocyathan boundstone dominated by calcimicrobes (Epiphyton and Renalcis), neomorphosed microbial crusts and crypto-fibrous calcite crusts around mainly regular archaeocyaths such as Polycoscinus scutatus, where encrusting ?sponges are accessory. The published stratigraphical and palaeocurrent data from the Dwyka Group, as well as published ages and the tectonothermal history of the underlying Nama Group, have been used in this study to establish the provenance of these lower Cambrian cobbles. The archaeocyaths from the Aranos Basin are: Favilynthus contractus, Ajacicyathus aequitriens, Kisasacyathus subacutus, Plicocyathus sp., Erismacoscinus endutus, Erismacoscinus ganigobisensis sp. nov., Erismacoscinus cf. stephensoni, Polycoscinus scutatus, Chabakovicyathus sp., Loculicyathidae gen. et sp. indet., Dictyocyathus aranosensis sp. nov., Graphoscyphia sp., Archaeopharetra irregularis and Syringocnema favus. The presence of Syringocnema favus and other species allows us to determine a precise upper Stage 3 or early-mid Botoman age for the erratic cobbles of the Dwyka Group from the Aranos Basin. The Namibian fauna reported here provides a key record in Cambrian palaeobiogeography based on archaeocyathan assemblages. The archaeocyath faunal assemblage from the Dwyka cobbles is diverse and has an analogous richness to allochthonous assemblages from Permo-Carboniferous glacigenic deposits of other localities in south-western Gondwana. Thus, the archaeocyath-bearing cobbles from the Dwyka Group of Namibia could provide a better understanding of the different palaeo-ice flow direction models of the Permo-Carboniferous glacial deposits in future palaeogeographical reconstructions of south-western Gondwana.
http://zoobank.org/urn:lsid:zoobank.org:pub:0365EE4C-D7FA-4C09-89DF-27C9E1F55511

本研究首次报道了纳米比亚阿拉诺斯盆地（Aranos Basin）德维卡群（Dwyka Group）石炭纪冰碛沉积中的灰岩砾石内产出的下寒武统古杯动物群（archaeocyathan fauna）。通过对17件大、中、标准薄片开展岩石学与微相分析，识别出两类特征鲜明的灰岩砾石相：（a）生物碎屑灰泥岩-颗粒灰岩（bioclastic wackestone-packstone），包含软舌螺类（hyoliths）、三叶虫（trilobites）、开腔骨类（chancelloriids）、海百合类（crinoids）及古杯类；（b）以钙化微生物（calcimicrobes，包括附枝藻属Epiphyton与Renalcis属）、新改造的微生物结壳，以及主要附着于规整古杯类（regular archaeocyaths，如盔壳古杯Polycoscinus scutatus）周围的隐纤维方解石结壳为主的微生物-古杯粘结岩（microbial-archaeocyathan boundstone），其中附生？海绵类为次要组分。本研究结合已发表的德维卡群地层与古流数据，以及下伏纳马群（Nama Group）的测年数据与构造热演化历史，厘定了这批下寒武统砾石的物源。本次在阿拉诺斯盆地发现的古杯类物种包括：Favilynthus contractus、Ajacicyathus aequitriens、Kisasacyathus subacutus、Plicocyathus sp.、Erismacoscinus endutus、Erismacoscinus ganigobisensis sp. nov.、Erismacoscinus cf. stephensoni、Polycoscinus scutatus、Chabakovicyathus sp.、Loculicyathidae gen. et sp. indet.、Dictyocyathus aranosensis sp. nov.、Graphoscyphia sp.、Archaeopharetra irregularis及Syringocnema favus。基于Syringocnema favus与其他物种的产出特征，可精准确定阿拉诺斯盆地德维卡群中这些漂砾的时代为寒武系第3阶上部或博托马阶（Botoman）早-中期。本次报道的纳米比亚古杯动物群，为基于古杯动物组合的寒武纪古生物地理学研究提供了关键记录。德维卡群砾石中的古杯动物群组合具有较高多样性，其丰富度与冈瓦纳西南部（Gondwana）其他地区二叠-石炭纪（Permo-Carboniferous）冰碛沉积中的异地组合（allochthonous assemblages）相当。因此，纳米比亚德维卡群中产古杯类的砾石，可为未来冈瓦纳西南部古地理重建中，厘清二叠-石炭纪冰碛岩的不同古冰流方向模型提供新的研究视角。
http://zoobank.org/urn:lsid:zoobank.org:pub:0365EE4C-D7FA-4C09-89DF-27C9E1F55511