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1. Grooving in the midcontinent: A tectonic origin for the mysterious striations of L’Anse Bay, Michigan, USA

   https://doi.org/10.1130/GES02603.1  

   Alemu, Tadesse B; Hodgin, Eben B; Swanson-Hysell, Nicholas L (July 2023, Geosphere)

   A striated surface is present at an erosional unconformity between foliated Paleoproterozoic Michigamme Formation and fluvial conglomerate and sandstone of the Neoproterozoic Jacobsville Formation exposed at L’Anse Bay (Michigan, USA). These striations have been interpreted to be the result of ice flow in either the Proterozoic, the Pleistocene, or the modern. Recently, the glacial origin interpretation for this striated surface has been used to argue that it may be related to ca. 717–635 Ma Cryogenian snowball Earth glaciation. This interpretation would make the surface a rare example of a Neoproterozoic glacial pavement, with major chronostratigraphic implications that in turn impose constraints on the timing of intracratonic erosion related to the formation of the Great Unconformity. In this contribution, we present new observations showing that the surface is a tectonic slickenside caused by largely unconformity-parallel slip along the erosional unconformity. We document structural repetition of the Michigamme-Jacobsville contact with associated small-scale folding. The unconformity-parallel slip transitions into thrust faults that ramp up into the overlying Jacobsville Formation. We interpret that the surface records contractional deformation rather than ancient glaciation, recent ice movement, or recent mass wasting. The faulting likely occurred during the Rigolet phase of the Grenvillian orogeny, which also folded the Jacobsville Formation in the footwall of the Keweenaw fault. 

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2. Detecting upland glaciation in Earth’s pre-Pleistocene record

   https://doi.org/10.3389/feart.2022.904787  

   Soreghan, Gerilyn S.; Pfeifer, Lily S.; Sweet, Dustin E.; Heavens, Nicholas G. (August 2022, Frontiers in Earth Science)

   Earth has sustained continental glaciation several times in its past. Because continental glaciers ground to low elevations, sedimentary records of ice contact can be preserved from regions that were below base level, or subject to subsidence. In such regions, glaciated pavements, ice-contact deposits such as glacial till with striated clasts, and glaciolacustrine or glaciomarine strata with dropstones reveal clear signs of former glaciation. But assessing upland (mountain) glaciation poses particular challenges because elevated regions typically erode, and thus have extraordinarily poor preservation potential. Here we propose approaches for detecting the former presence of glaciation in the absence or near-absence of ice-contact indicators; we apply this specifically to the problem of detecting upland glaciation, and consider the implications for Earth’s climate system. Where even piedmont regions are eroded, pro- and periglacial phenomena will constitute the primary record of upland glaciation. Striations on large (pebble and larger) clasts survive only a few km of fluvial transport, but microtextures developed on quartz sand survive longer distances of transport, and record high-stress fractures consistent with glaciation. Proglacial fluvial systems can be difficult to distinguish from non-glacial systems, but a preponderance of facies signaling abundant water and sediment, such as hyperconcentrated flood flows, non-cohesive fine-grained debris flows, and/or large-scale and coarse-grained cross-stratification are consistent with proglacial conditions, especially in combination with evidence for cold temperatures, such as rip-up clasts composed of noncohesive sediment, indicating frozen conditions, and/or evidence for a predominance of physical over chemical weathering. Other indicators of freezing (periglacial) conditions include frozen-ground phenomena such as fossil ice wedges and ice crystals. Voluminous loess deposits and eolian-marine silt/mudstone characterized by silt modes, a significant proportion of primary silicate minerals, and a provenance from non-silt precursors can indicate the operation of glacial grinding, even though such deposits may be far removed from the site(s) of glaciation. Ultimately, in the absence of unambiguous ice-contact indicators, inferences of glaciation must be grounded on an array of observations that together record abundant meltwater, temperatures capable of sustaining glaciation, and glacial weathering (e.g., glacial grinding). If such arguments are viable, they can bolster the accuracy of past climate models, and guide climate modelers in assessing the types of forcings that could enable glaciation at elevation, as well as the extent to which (extensive) upland glaciation might have influenced global climate. 

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3. Rapid morphological change in the Late Triassic bivalve Monotis from Aotearoa New Zealand: Environmental or Biological Drivers?

   Clement, A; Tackett, L; Sclafani, J; Christie, M (October 2023, Geological Society of America Abstracts with Programs)

   In the South Island of Aotearoa (New Zealand), the preservation of biogenic carbonate in Late Triassic sedimentary rocks is rare to non-existent; however, differential preservation modes between common phyla are commonly observed and serve to elucidate the stratigraphic and diagenetic history of these often poorly- exposed immature sandstone units. The Taringatura Group sandstones from Southland and Otago range from sandy siltstones to silty arkosic sandstones that commonly host molluscan and brachiopod macrofossils as well as rare echinoderms, bryozoans, and foraminifera. Additionally, there is a hypothesized unconformity between the lower Oretian and Otamitan age (227.7–217.0 Ma) and the overlying Warepan age (217–208.5 Ma) deposits indicated by an abrupt change in composition, grain size, and fossil assemblage. Molluscs from the Oretian and Otamitan deposits exhibit fine-detail preservation on external and internal molds. Thin-shelled taxa, such as Halobia, exhibit some shell replacement by clay minerals, likely from the dissolution of feldspars in the surrounding rock. Conversely, larger and thicker-shelled brachiopods and bivalves such as Manticula and Hokonuia do not present as casts. When preserved, foraminifera and rare bryozoans are typically silicified. The overlying Warepan sandstone beds frequently contain fossils of the bivalve Monotis which exhibit a similar preservation style to older molluscs, though lacking clay minerals. Presently, the fossiliferous Taringatura sandstones exhibit low porosity and low permeability, as is expected from the subsequent compaction of sandstones after burial. However, the dissolution of biogenic carbonate implies a past permeability. The presence of clay minerals in Oretian and Otamitan fossils may indicate a period of subaerial exposure and infiltration of meteoric water prior to the deposition of Warepan units. Notably, clay replacement occurs more frequently in the thinnest fossils. Original carbonate material may have persisted for longer in the more robust taxa, allowing them to resist most deformation from compaction prior to the final loss of carbonate. Differential diagenesis of biogenic carbonates supports the existence of a significant unconformity between Otamitan and Warepan units in the Taringatura sandstones. 

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4. Glaciogenic sedimentary infill of late Paleozoic glacial paleovalleys of the Kaokoveld, Northwest Namibia

   https://doi.org/10.1130/abs/2019AM-340908  

   Rosa, E.L.; Isbell, J. L. (September 2019, Geological Society of America Abstracts with Programs)

   In the Kaokoveld (NW Namibia), several modern river valleys are exhuming late Paleozoic glacial valleys cut onto Precambrian fold belts. They represent one of the most prominent late Paleozoic exhumed glacial landscapes and are widely considered to have been carved by outlet glaciers that drained the Windhoek Ice Sheet and fed marginal lobes that flowed into the Paraná Basin, southern Brazil. No detailed research exists on the glacial sedimentary fill of these valleys. Two study sites in the Khumib and Kunene rivers catchment were analyzed for depositional environments, glacial cyclicity, and relative timing of deposition recorded in the Dwyka Group. The Dwyka strata are confined within these valleys and dip up to 30 degrees outward away from the valley walls becoming horizontal near the axis of the valleys. Sedimentary units include: 1) thick successions of diamictite- and conglomerate-bearing clinoforms containing boulders up to 2 m in diameter generated by sediment-laden meltwater, sediment gravity flows and iceberg rainout with intraformational grooved surfaces generated by coeval iceberg scour; 2) laminated, fine-grained sandstone/mudstone rhythmites with dropstones, dump structures, interbedded rainout diamictites and sole mark-bearing finegrained massive to current-rippled sandstones (turbidites). These units were deposited in the distal zones of a subaqueous outwash system; 3) folded and sheared intervals of the above facies interpreted as having been deformed subglacially and in ice-marginal settings during ice advance. Ice advance is indicated by the occurrence of overlying erosional based conglomerates interpreted as outwash deposits; and 4) a capping succession of fine-grained massive, horizontally laminated, and current-rippled sandstones with sole marks and laminated rhythmites with convolute bedding interpreted as turbidity flow deposits generated following glaciers retreat. The stacking of these units is consistent with the occurrence of oscillating margins of temperate, tidewater termini of fast flowing ice with deposition occurring in morainal banks or grounding-zone wedges during at least two glacial advance-retread cycles. The morphology of the valleys and their sedimentary infill suggest that they were shaped by ice streams during the Late Paleozoic Ice Age. 

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5. PROVENANCE OF THE UPPERMOST CARBONIFEROUS–LOWER TRIASSIC SANDSTONES, BOGDA MOUNTAINS, NW CHINA: IMPLICATION ON THE LATE PALEOZOIC TECTONIC HISTORY OF THE SOUTHERN CENTRAL ASIAN OROGENIC BELT

   https://doi.org/10.1130/abs/2024AM-401642  

   Yang, Wan; Wu, Sixuan (January 2024, Geological Society of America)

   The Permian-Triassic time is a significant stage in the Paleozoic continental amalgamation and Cenozoic orogenic reactivation of southern Central Asian Orogenic Belt (CAOB). Field, petrographic, and detrital zircon U-Pb geochronological data of the uppermost Carboniferous–Lower Triassic sandstones from 3 sections in Bogda Mountains, greater Turpan-Junggar basin, NW China, are used to decipher the tectonic history. The sections are Tarlong-Taodonggou (TT) and Zhaobishan (ZBS) in the south and Dalongkou (DLK) in the north, 100 km apart and ~7,000 m in total thickness. Four petrofacies of 229 sandstones and U-Pb dates of 3505 zircons of 35 sandstones form the basis for interpretation. During Gzhelian–Asselian, andesite and basalt are the major source lithologies in TT. Zircon ages peak at ~300 Ma. During Sakmarian–Kungurian, basalt and andesite are the main source rocks in TT and ZBS; and zircon ages of both areas peak at ~300 Ma. The Roadian–Wordian is represented by a regional unconformity. The Guadalupian source lithology and zircon date show a major change. Andesite is the common and rhyolite and basalt the minor source lithologies for TT and DLK; but rhyolite for ZBS. A unimodal peak at ~305 Ma occurs in TT; but two peaks at 305 and 455 Ma with common Precambrian dates in ZBS; and peaks of 310–295 Ma in DLK. During Wuchiapingian–mid Olenekian, andesite and rhyolite are the common source lithologies for TT and DLK; but rhyolite as the primary volcanic lithology for ZBS. In TT, Wuchiapingian- Induan samples have a major age peak at ~300 Ma, and an Olenekian sample has two peaks at ~300 and ~450 Ma. In ZBS, the age pattern is similar to that of the Guadalupian sample. In DLK, samples have a major peak at ~310 Ma and a minor peak at ~450 Ma. The comparable age clusters identified by multi-dimensional scaling indicate that North Tianshan is the source for TT and ZBS during the latest Carboniferous–early Permian. But in mid Permian, south Central Tianshan became the main source solely to ZBS. During late Permian–Early Triassic, both north and central Tianshan became the common sources to all three areas due to enhanced denudation. The source change in mid-Permian across a regional unconformity is synchronous with Paleo-Asian Ocean closure and arc-continent and continent-continent collisions, which occurred along the southern margin of Turpan- Junggar basin no later than Guadalupian. 

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