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Chapter 14 Glaciogenic and associated strata of the Otavi carbonate platform and foreslope, northern Namibia: evidence for large base-level and glacioeustatic changes - Photograph supplement

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Title: Chapter 14 Glaciogenic and associated strata of the Otavi carbonate platform and foreslope, northern Namibia: evidence for large base-level and glacioeustatic changes - Photograph supplement
Author: Hoffman, Paul F.; Crockford, Peter
Abstract: Companion photographs to the book chapter Hoffman, Paul F. 2011. “Glaciogenic and associated strata of the Otavi carbonate platform and foreslope, northern Namibia: evidence for large base-level and glacioeustatic changes.” In: Arnaud, E., Halverson, G. P. and Shields-Zhou, G. (eds) The Geological Record of Neoproterozoic Glaciations. Geological Society, London, Memoirs, 36, 195-209.
URI: http://hdl.handle.net/10214/9376
Date: 2011
Rights: Attribution-NonCommercial-NoDerivs 2.5 Canada
Rights Holder: The copyright of the photos rests with the author(s) of the chapter or, where stated, the person who took the photo.


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Ch14_Photo_1_Hoffman.jpg 882.0Kb JPEG image Thumbnail Sturtian glacial record on the western Otavi platform, where north-south crustal stretching occurred before, during and after the glaciation. Nearly 450 m of Chuos Formation (Ac), consisting of polymictic (cover and basement-derived) diamictite with subordinate sandstone and siltstone, fills a palæovalley incised in tilted, mixed shallow-water carbonates and clastics of the Ombombo Subgroup (Ob through Ok) near Omutirapo (19°07'18"S, 13°56'06"E), eastern Kaoko belt. Note onlap of Chuos strata against the palæovalley wall, viewed looking northwestward. Units (older to younger): Ob, Beesvlakte Formation, marlstone; Od, Devede Formation, peritidal dolostone; Odt, Tungussia stromatolite member of Od; Ok, Okakuyu Formation, clastics; Ac, Chuos Formation, diamictite with subordinate sand- and siltstone; Ar, Rasthof Formation, ‘cap’ limestone and sublittoral microbial dolostone (Photo credit: Paul Hoffman).
Ch14_Photo_2_Hoffman.jpg 1.249Mb JPEG image Thumbnail Marinoan glacial record on the distal foreslope of the Otavi platform near Fransfontein (20°11'52"S, 15°00'54"E). A coarsening-upward sequence of carbonate turbidites and oolite-clast debrites (Franni-aus Member) disconformably overlies siltstone (Narachaams Member) and is interpreted as a glacioeustatic falling-stand wedge. It is disconformably overlain by the main body of the Ghaub Formation, a complex of massive carbonate-clast diamictites intercalated with thinner intervals of well-stratified detrital carbonate. The top of the diamictite complex is sharply overlain by a fining-upward sequence of Fe-rich dolostone turbidites and debrites (Bethanis Member of the Ghaub Formation), which is choked with ice-rafted debris of all sizes and locally contains westerly-directed starved ripples (contourites). The Bethanis Member is sharply but conformably overlain by the Keilberg Member of the Maieberg Formation, a pale-coloured micropeloidal ‘cap’ dolostone with small-scale cross-stratification and locally giant wave ripples. The maximum flooding zone of the Maieberg Formation (cap-carbonate sequence) is a marly recessive interval a few metres above the Keilberg Member. In the distal foreslope setting, the top of the Maieberg Formation is correlated isotopically with a major sequence boundary on the platform. The Karibib Formation comprises distal foreslope equivalents of the Elandshoek and Hüttenberg formations on the platform. The main Ghaub Formation and its basal disconformity provide evidence for grounded ice on the distal foreslope of the Otavi carbonate platform. The Keilberg Member is the only unit on the distal foreslope deposited above prevailing wave-base (Photo credit: Paul Hoffman)
Ch14_Photo_3_Hoffman.jpg 833.2Kb JPEG image Thumbnail Well-stratified detrital carbonate of periglacial marine origin near the base of the main Ghaub Formation on the distal foreslope east of Fransfontein (20°11'43"S, 15°05'56"E). There are three depositional components: (1) parallel-laminated, normal and reverse graded, silt (tan colour), interpreted as fallout from hypopycnal meltwater plumes; (2) graded beds of sandy turbidite (blue-grey colour) and (3) outsize clasts, localized in plume fallout and absent from turbidites, consistent with a steady rate of clast liberation from icebergs or shelf ice (Photo credit: Paul Hoffman).
Ch14_Photo_4_Hoffman.jpg 1.139Mb JPEG image Thumbnail Ice-rafted dropstone of oolitic limestone in well-stratified periglacial detrital carbonate near the base of the main Ghaub Formation on the distal foreslope east of Fransfontein (20°11'43"S, 15°05'56"E). Impact of the dropstone deformed underlying beds and ejected a doubly-folded flap (arrow) of the contemporary surface bed. Note depositional onlap and drape of subsequent beds (Photo credit: Paul Hoffman).
Ch14_Photo_5_Crockford.jpg 1.567Mb JPEG image Thumbnail Channel of sorted and stratified detrital carbonate beneath a mappable reactivation surface (not shown) within a compound massive diamictite in the upper part of the main Ghaub Formation near Fransfontein (20°11'53"S, 15°01'11"E). The channel-fill consists of a lower onlapping sequence and an upper downlapping sequence indicating southwesterly-directed flow of inferred meltwater beneath grounded ice, from which the massive diamictite was deposited (Photo credit: Peter Crockford).
Ch14_Photo_6_Hoffman.jpg 1.447Mb JPEG image Thumbnail The Bethanis Member (Tb) of the Ghaub Formation on the distal foreslope near Fransfontein (20°11'58"S, 15°01'04"E) forms a 10-m-thick blanket of chocolate-brown, fining-upward debrites and turbidites, choked with polymictic ice-rafted debris of all sizes. It is conformably overlain by the Keilberg Member (Tk) of the Maieberg Formation, a transgressive syndeglacial dolopelarenite (‘cap’ dolostone), and is underlain by a complex of massive carbonate diamictites with subordinate well-stratified detrital carbonate of the main Ghaub Formation (Tg). The Bethanis Member records the final collapse of the ice sheet on the Otavi platform and its distinctive chocolate-brown colour is attributed to oxidation of dissolved Fe following deglaciation of the tropical ocean. The Bethanis Member everywhere blankets the Ghaub Formation except on Duurwater moraine, which like the platform and upper foreslope were apparently emergent when the Bethanis Member was deposited (Photo credit: Paul Hoffman).
Ch14_Photo_7_Hoffman.jpg 1.076Mb JPEG image Thumbnail Lodegement facies of the Ghaub Formation on the inner part of the Otavi platform near Ombaatjie (19°19'04"S, 13°59'33"E). Up to 2.0 m of lodgement tillite and associated sorted facies occur discontinuously on the inner platform, but are entirely absent on the raised outer platform and upper foreslope. Here, laminated siltstone with rafted clasts and doloclast breccia disconformably overlie stromatolitic dolostone of the Ombaatjie Formation (Ab). A scour surface separates lodegement facies (Tg) from low-angle cross-stratified dolopelarenite of the Keilberg Member (Tk). Length of hammer is 32 cm (Photo credit: Paul Hoffman).
Ch14_Photo_8_Hoffman.jpg 1.447Mb JPEG image Thumbnail Marinoan glacial record on the inner part of the Otavi platform near Ombaatjie (19°18'39"S, 13°59'52"E). Discontinuous lodgement facies (Photo 7) on the glacial surface are sharply overlain by 15 m of dolopelarenite (Keilberg Member) with tubestone stromatolite in the lower (but not basal) part and giant wave ripples in the upper part. The Keilberg Member is gradationally overlain by marly limestone rhythmite (maximum flooding zone) and a highstand sequence comprised of pink and grey limestone rhythmites with large-scale low-angle crossbedding, grey dolostone rhythmite and dolostone grainstone capped by a tepee-structured and heavily silicified subaerial exposure surface. Above the exposure surface is a 500-m thick succession of meter-scale microbialaminite- and grainstone-capped cycles of the Elandshoek Formation. The Maieberg Formation comprises the full postglacial depositional sequence (‘cap-carbonate sequence’ of Hoffman & Schrag 2002) and the Keilberg Member (‘cap dolostone’) is its transgressive systems tract. The section shown is typical of the Otavi platform, except that the Keilberg Member thickens to <80 m on the raised outer part of the platform. Thus, the cap dolostone is thickest where glacial deposits are absent, and thinnest on the distal foreslope where glacial deposits are best developed. For photos of Keilberg Member lithofacies see Chapter 5 (Photo credit: Paul Hoffman).

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Attribution-NonCommercial-NoDerivs 2.5 Canada Except where otherwise noted, this item's license is described as Attribution-NonCommercial-NoDerivs 2.5 Canada