Teaching Slides: Glacial Meltdowns & Cap Carbonates
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4.2: Contact between the younger Cryogenian cap dolostone (CD) and proglacial marine deposits (DF, debris flows; IRD, ice-rafted debris) of the Ghaub Formation (635 Ma), near Narachaams se Pos, southern foreslope of the Otavi Group, Namibia.



4.3: Contact between the younger Cryogenian Noonday cap dolostone (CD) and glacial marine diamictite of the Surprise Member (SM), Goler Wash, Panamint Range, Death Valley area, California.



4.4: Younger Cryogenian cap dolostone (Ravensthroat Formation) in the Shale Lake area, Mackenzie Mountains, northern Canadian Cordillera.



4.5: Detail of younger Cryogenian glacial and post-glacial strata in the Arctic Red River section, Mackenzie Mountains, northern Canadian Cordillera.



4.6: Younger Cryogenian glacial and post-glacial strata in the Tillite Group of the East Greenland Caledonides.



4.7: Younger Cryogenian glacial and post-glacial strata in the Polarisbreen Group of the East Svalbard Caledonides.



4.8: Contact between the younger Cryogenian (Marinoan) cap dolostone (Nuccaleena Formation) and the Elatina Formation glacial-marine diamictite, Enorama Creek (GSSP site), Flinders Range, South Australia.



4.9: Contact between the younger Cryogenian (Marinoan) cap dolostone (Mirassol Formation) and the Puga Formation glacial diamictite, Mirassol d’Oeste, Mato Grosso, Brazil.



4.10: Contact (left) between the younger Cryogenian (Marinoan) cap dolostone (Hadash Formation) and the Fiq Formation glacial diamictite, Wadi Hajir, Oman Mountains, Oman. Characteristic structures in younger Cryogenian cap carbonates (right): giant wave ripples in the Keilberg cap dolostone, NW Namibia, sea-floor cements (former aragonite crystal fans) in the Hayhook cap limestone, NW Canada.



4.11: Basal Ediacaran cap carbonate (lower Doushantuo Formation) in the Yangtze Gorges area of South China. A thin clay-rich layer (red line) occurs between the Nantuo glacigenic diamictite and the base of the cap dolostone (units C1 and C2 of Jiang et al., 2005). An homologous layer in NW Canada contains parts-per-billion levels of platinum group elements, or PGEs (Bernhard Peucker-Ehrenbrink, personal communication). A thin volcanic tuff (green line) lying directly on top of the cap dolostone has been dated at 635.2 0.5 Ma by the ID-TIMS U-Pb zircon method (Condon et al., 2005), placing a tight minimum constraint on the end of the Nantuo glaciation. The cap dolostone is characterized by penecontemporaneous brecciation and barite precipitation, possibly in the vadose zone. Overlying, deeper-water, marly limestone corresponds to unit C3 of Jiang et al., 2005).



4.12: Stratigraphic comparison of younger Cryogenian (Marinoan) cap carbonates in Australia, Brazil, Canada, Svalbard, NW Namibia (Congo craton), SW Namibia (Kalahari craton), and Svalbard.



4.13: Generic transgressive tract of the younger Cryogenian (Marinoan) cap-carbonate sequence.



4.14: Characteristic structures in younger Cryogenian (Marinoan) cap dolostones: (below) graded and reverse-graded peloids with low-angle cross lamination, Ravensthroat Formation, NW Canada (inset: Tereeken cap dolostone, Quruqtagh, NW China); plumb (tubular) stromatolite, Keilberg Member, NW Namibia.



4.15: Detail of peloidal cap dolostone with low-angle cross-lamination, Ravensthroat Formation, NW Canada. Macropeloidal texture is characteristic of cap dolostones in Brazil, Canada, NW China, Namibia and Svalbard.



4.16: Plumb (tubular) stromatolite in the Keilberg Member cap dolostone, Namibia.



4.17: Gutter stromatolite overlain by giant wave ripples in the Mirassol cap dolostone, SW Brazil.



4.18: Giant wave ripples in the Keilberg cap dolostone (below), NW Namibia and the Raventhroat cap dolostone (above), NW Canada (cross-section, left; synoptic morphology, right).



4.19: Cross-section of the crest of a giant wave ripple in the Ravensthroat cap dolostone, NW Canada, showing characteristic bidirectional laminae and chevron-like up-building. Coin is 2 cm in diameter.



4.20: Giant wave ripples in the Ravensthroat cap dolostone, NW Canada, showing characteristic wavelength ~1.5 m and synoptic relief ~0.35 m.



4.21: Azimuthal orientations of giant wave ripples in the Ravensthroat cap dolostone at different locations near the contemporaneous shelf-slope break in the Mackenzie Mountains, northern Canadian Cordillera. Orientations in Namibia are also subnormal to the shelf-slope break.



4.22: Characteristic development of giant wave ripples in younger Cryogenian (Marinoan) cap dolostones in Canada, Namibia, Svalbard, Australia and Brazil.



4.23: Giant wave ripples in Marinoan cap dolostones in SW Brazil (cross-section and morphological cast) and central Australia.



4.24: Giant wave ripple in the Landrigan (Marinoan?) cap dolostone, Louisa Downs, Kimberley Region, Western Australia.



4.25: Tentative paleogeographic reconstruction of continents in the southern hemisphere at ~650 Ma (after Murphy et al., 2004) showing regional average azimuthal paleo-orientations (red bars) of the crestlines of giant wave ripples in 635-Ma cap dolostones in NW Canada, East Svalbard, SW Brazil and NW Namibia (Paul Hoffman unpublished observations).



4.26: True tepee structure (cross-section of pressure-ridge caused by the crystallization force of intergranular evaporite) marking the subaerial exposure surface at the top of the cap dolostone above continental tillite of the younger Cryogenian(?) Jbéliat (Bthaat Ergil) Group, Adrar, Mauritania, West Africa.



4.27: Sea-floor barite at the top of the Raventhroat cap dolostone, Shale Lake section, Mackenzie Mountains, NW Canada. Barite, variably pseudomorphosed by calcite), occurs throughout the southern Mackenzie Mountains at the precise transition from peloidal dolostone to calcite micrite (with former aragonite crystal fans).



4.28: Thin section of sea-floor barite from the Ravensthroat cap dolostone, NW Canada. Note growth layers in 1st-generation barite (b1) "fingers", and step-outs of 2nd-generation barite (b2) on laminae of peloidal dolostone (dm) deposited between the barite fingers. The b1 barite fingers must therefore have projected above the sediment-water interface and were precipitated directly from seawater, unlike most sedimentary barite which precipitates from pore-waters during diagenesis.



4.29: Sea-floor barite crust drapes a giant wave ripple at the top of the Ravensthroat cap dolostone, Shale Lake section, Mackenzie Mountains, Canada. Barite is directly overlain by micritic limestone with crystal fans of former aragonite sea-floor cement.



4.30: Transitions from Ravensthroat cap dolostone to Hayhook cap limestone in which the sea-floor barite cements (pseudo-stromatolites) have been replaced by calcite (below) and a solution-collapse mini-breccia (above). Barite replacement and dissolution probably occurred under the influence of reducing groundwaters percolating downward from the directly overlying black pyritic shale (>700 m thick) of the Sheepbed Formation. Note palimpsest growth laminae in the calcitized barite pseudo-stromatolites.



431: Crystal fans of pseudomorphosed aragonite sea-floor cement in micritic limestones above the younger Cryogenian cap dolostones in Namibia (below) and Canada (above).



4.32: Former aragonite crystal fans in calcite micrite of the Hayhook Formation in the younger Cryogenian cap-carbonate sequence, Moose Horn River section, Mackenzie Mountains, Canada.



4.33: Stratigraphic sequence associated with the younger Cryogenian glaciation (Stelfox Member, Ice Brook Formation) in the southern Mackenzie Mountains, Canada.



4.34: Shale Lake section of the Cryogenian-Ediacaran transition in the Mackenzie Mountains, Canada.



4.35: Ravensthroat cap dolostone and Hayhook liimestone of the younger Cryogenian (Marinoan) cap-carbonate sequence, Moose Horn River section, Mackenzie Mountains, Canada.



4.36: Stratigraphic sequence associated with the younger Cryogenian glaciation at the edge of the Otavi Group carbonate platform in NW Namibia.



4.37: Complete post-glacial cap-carbonate sequence (Maieberg Formation) associated with the younger Cryogenian glaciation on the Otavi Group carbonate platform near Obaatjie, NW Namibia. TST (CD), Keilberg Member cap dolostone; MFZ, limestone rhythmite of the maximum flooding stage; HST, flaggy limestone and dolostone grainstone of the highstand systems tract.






4.38: Surtian and Marinoan cap-carbonate sequences in clastic-dominated, mixed clastic-carbonate, and carbonate-dominated successions on tectonically-subsiding shelves and platforms.



4.39:Carbon (δ13C) and oxygen (δ18O) isotopic profiles of the younger Cryogenian cap-carbonate sequence and the pre-glacial Trezona anomaly, Otavi Group carbonate platform near Ombaatjie, NW Namibia.



4.40: Marly limestone rhythmite in the maximum flooding stage of the younger Cryogenian (Marinoan) cap-carbonate sequence (Maieberg Formation), Otavi Group carbonate platform near Ombaatjie, NW Namibia.



4.41: Sedimentary facies and carbon isotopes of the Keilberg cap dolostone across the Otavi carbonate platform in northern Namibia.



4.42: Carbon/oxygen isotope cross-plot for Holocene "organogenic" secondary dolomites (Mazzullo, 2000). Large variations in carbon isotopes are caused by production of 13C-enriched CO2 during methanogenesis and 13C-depleted CO2 during bacterial sulfate reduction. This contrasts with tightly clustered isotopic data from 635-Ma cap dolostones in Namibia, Svalbard and NW Canada, which are interpreted to reflect equilibrium with contemporaneous seawater.



4.43: Regional variation in thickness, facies and carbon isotopic composition of the younger Cryogenian (Marinoan) cap dolostone on the Otavi Group carbonate platform (above) and foreslope (below), Namibia.



4.44:Transient ocean hyperstratification accompanying the meltdown of a snowball earth.



4.45: Early stage in the flooding of the Otavi Group carbonate platform accompanying the meltdown of the younger Cryogenian snowball earth in 635 Ma.



4.46: Later stage in the flooding of the Otavi Group carbonate platform accompanying the meltdown of the younger Cryogenian snowball earth in 635 Ma.



4.47: The complete cap-carbonate sequence (Rasthof Formation) of the older Cryogenian (Sturtian) glaciation (Chuos Formation), Otavi Group platform, near Omukutu, upper Hoanib River, NW Namibia. Top of cap-carbonate sequence is the first subaerial exposure surface above the glacial.



4.48: Sea-floor cement (pseudomorphosed aragonite crystal fans) in the Sete Lagoas cap-carbonate sequence above the older Cryogenian(?) glacials (Macaubas Formation) in the Bambuí Group on the Sao Francisco craton, SE Brazil.