POTENTIAL ICE CRYSTAL MARKS FROM PENNSYLVANIAN–PERMIAN EQUATORIAL RED-BEDS OF NORTHWEST COLORADO, U.S.A.

ABSTRACT The Pennsylvanian–Permian Maroon Formation of northwest Colorado is an up to 4,600 m thick succession of mainly siliciclastic continental red-beds deposited in equatorial intermontane basins of the Ancestral Rocky Mountains. Sedimentary surfaces of fluvio-lacustrine to eolian siltstones and fine-grained sandstones from various stratigraphic levels within the Maroon Formation preserve cm-sized straight to gently curved sediment-filled acicular structures referred to five morphological groups: single, branched, stellate, rosette, and bunched. Depositional environment, shape, and size of the structures are most similar to ice crystal marks that result from freezing of water-saturated fine-grained substrate at the sediment-air interface. They differ from other syngenetically produced crystals and crystal pseudomorphs in sedimentary rocks mainly by crystal shape and environmental conditions. The potential ice crystal marks of the Maroon Formation are notable for the fidelity and morphological diversity of the crystal casts and could be a key for the understanding of similar but hitherto often only called enigmatic structures of the sedimentary rock record. The ice crystal mark occurrences in the Maroon Formation suggest that night frost affected lower elevation equatorial areas during the climax of the Late Paleozoic Ice Age and may stimulate research on evolutionary adaptations of early terrestrial biota to overcome significant air temperature fluctuations.

Carboniferous biostratigraphy of rugose corals

Rugose corals are one of the major fossil groups in shallow-water environments. They played an important role in dividing and correlating Carboniferous strata during the last century, when regional biostratigraphic schemes were established and may be useful for long-distance correlation. Carboniferous rugose corals document two evolutionary events. One is the Tournaisian recovery event, with abundant occurrences of typical Carboniferous rugose corals such as columellate taxa and a significant diversification of large, dissepimented corals. The other is the changeover of rugose coral composition at the mid-Carboniferous boundary, which is represented by the disappearance of many large dissepimented taxa with complex axial structures and the appearance of typical Pennsylvanian taxa characterized by compound rugose taxa. The biostratigraphic scales for rugose corals show a finer temporal resolution in the Mississippian than in the Pennsylvanian, which was probably caused by the Late Paleozoic Ice Age that resulted in glacial-eustatic changes and a lack of continuous Pennsylvanian carbonate strata. The Pennsylvanian rugose corals are totally missing in the Cimmerian Continent. High-resolution biostratigraphy of rugose corals has so far only achieved in few regions for the Mississippian time scale. In most regions, more detailed taxonomic works and precise correlations between different fossil groups are needed.

Low-latitude climate change linked to high-latitude glaciation during the Late Paleozoic Ice Age: evidence from the terrigenous detrital kaolinite

The Late Paleozoic Ice Age (LPIA; ca. 360–260 million years ago) was one of the most significant glacial events in Earth history that records cycles of ice advance and retreat in southern high-latitude Gondwana and provides a deep-time perspective for climate-glaciation coevolution. However, climate records from the LIPA are poorly understood in low latitudes, particularly in the North China Plate (NCP) on the eastern Palaeo-Tethys. We address this through a detailed mineralogical study of the marine-continental sedimentary succession in the Yuzhou Coalfield from the southern NCP in which we apply Zircon U-Pb dating, biostratigraphy, and high-resolution clay mineral composition to reconstruct latest Carboniferous to early Permian chronostratigraphy and climate change. The Benxi, Taiyuan, and Shanxi formations in the study area are assigned to the Gzhelian, Asselian-Artinskian, and Kungurian-Roadian stages respectively and the Carboniferous Permian lithostratigraphy across NCP recognized as widely diachronous. Detrital micromorphology of kaolinite under scanning electron microscopy and illite crystallization indicates kaolinite contents to be a robust proxy for palaeoclimate reconstruction. Kaolinite data show alternating warm-humid and cool-humid climate conditions that are roughly consistent with the calibrated glacial-interglacial successions recognized in high-latitude eastern Australia, including the glaciations P1 (Asselian-early Sakmarian) and P2 (late Sakmarian-early Artinskian), as well as the climatic transition to glaciation P3 (Roadian). Our results indicate a comparatively cool-humid and warm-humid climate mode in low-latitude NCP during glacial and interglacial periods, and this is a significant step toward connecting climate change in low-latitudes to high-latitude glaciation during the LPIA.

Rochas glaciais sob o microscópio: microtexturas e microestruturas em fácies do Grupo Itararé

A Era Glacial Neopaleozoica (LPIA – Late Paleozoic Ice Age) é representada na Bacia do Paraná pelo Grupo Itararé, cujos estratos fornecem um registro glacial dominado por sucessões glácio-marinhas e raros intervalos deformados por geleiras. Depósitos glaciotectonizados possuem macroestruturas como dobras, falhas e zonas de cisalhamento geradas sob ou na margem da geleira, formados quando a margem glacial avança sobre os sedimentos. Poucos estudos existem em estruturas microscópicas glaciotectônicas em estratos pré-Cenozoicos. Este estudo tem como objetivo utilizar as microestruturas como ferramenta auxiliar na interpretação de depósitos sedimentares deformados em contexto glacial. Os resultados são utilizados para avaliar criticamente a aplicabilidade da micromorfologia no registro glacial pré-pleistocênico e como essas estruturas podem ser modificadas ao longo do tempo em resposta à litificação e à diagênese. O estudo combina dados de campo com a análise micromorfológica e microestrutural detalhada de 18 lâminas petrográficas de rochas coletadas na porção basal do Grupo Itararé no município de Balsa Nova. As amostras laminadas foram coletadas em litotipos diferentes, incluindo diamictitos, arenitos e lamitos, deformados por glaciotectonismo. As microestruturas encontradas incluem uniastrial e skelsepic plasmic fabrics, foliações tipo SC, plasma bandado, estruturas rotacionais, grain turbates,alinhamento de grãos, microzonas e microplanos de cisalhamento, clastos cisalhados, falhas, dobras, ‘boudins’, estruturas de escape de fluidos e intraclastos. Em fácies bem selecionadas, com pouca ou nenhuma matriz, ocorrem também estruturas tipicamente relacionadas à compactação e diagênese, como grãos fraturados, redução da porosidade primária, contatos suturados e estilolitos. Os resultados mostram grande variedade de assembleias possíveis para as microestruturas, não necessariamente relacionáveis com a posição dos sedimentos em relação ao gelo no momento da deformação. Além disso, a porcentagem de matriz na amostra desempenha papel fundamental na preservação de estruturas primárias e feições de deformação em sedimentos. Fácies com pouca matriz sofrem grandes mudanças texturais devido à diagênese, que podem se sobrepor ou obliterar completamente as microestruturas glaciais.

A lithofacies analysis of a South Polar glaciation in the Early Permian: Pagoda Formation, Shackleton Glacier region, Antarctica

ABSTRACT The currently favored hypothesis for Late Paleozoic Ice Age glaciations is that multiple ice centers were distributed across Gondwana and that these ice centers grew and shank asynchronously. Recent work has suggested that the Transantarctic Basin has glaciogenic deposits and erosional features from two different ice centers, one centered on the Antarctic Craton and another located over Marie Byrd Land. To work towards an understanding of LPIA glaciation that can be tied to global trends, these successions must be understood on a local level before they can be correlated to basinal, regional, or global patterns. This study evaluates the sedimentology, stratigraphy, and flow directions of the glaciogenic, Asselian–Sakmarian (Early Permian) Pagoda Formation from four localities in the Shackleton Glacier region of the Transantarctic Basin to characterize Late Paleozoic Ice Age glaciation in a South Polar, basin-marginal setting. These analyses show that the massive, sandy, clast-poor diamictites of the Pagoda Fm were deposited in a basin-marginal subaqueous setting through a variety of glaciogenic and glacially influenced mechanisms in a depositional environment with depths below normal wave base. Current-transported sands and stratified diamictites that occur at the top of the Pagoda Fm were deposited as part of grounding-line fan systems. Up to at least 100 m of topographic relief on the erosional surface underlying the Pagoda Fm strongly influenced the thickness and transport directions in the Pagoda Fm. Uniform subglacial striae orientations across 100 m of paleotopographic relief suggest that the glacier was significantly thick to “overtop” the paleotopography in the Shackleton Glacier region. This pattern suggests that the glacier was likely not alpine, but rather an ice cap or ice sheet. The greater part of the Pagoda Fm in the Shackleton Glacier region was deposited during a single retreat phase. This retreat phase is represented by a single glacial depositional sequence that is characteristic of a glacier with a temperate or mild subpolar thermal regime and significant meltwater discharge. The position of the glacier margin likely experienced minor fluctuations (readvances) during this retreat. Though the sediment in the Shackleton Glacier region was deposited during a single glacier retreat phase, evidence from this study does not preclude earlier or later glacier advance–retreat cycles preserved elsewhere in the basin. Ice flow directions indicate that the glacier responsible for this sedimentation was likely flowing off of an upland on the side of the Transantarctic Basin closer to the Panthalassan–Gondwanide margin (Marie Byrd Land), which supports the hypothesis that two different ice centers contributed glaciogenic sediments to the Transantarctic Basin. Together, these observations and interpretations provide a detailed local description of Asselian–Sakmarian glaciation in a South Polar setting that can be used to understand larger-scale patterns of regional and global climate change during the Late Paleozoic Ice Age.