WHY ARE CRYOGENIAN GLACIAL DEPOSITS ARE LOCALLY THICK, WHILE REGIONALLY THIN?: CASE STUDIES FROM THE OTAVI GROUP OF NAMIBIA

Partin & Sadler (Geology, 2016) show that Cryogenian glacial deposits are regionally thin but locally thick. The Otavi Group is a 2-km-thick blanket of dominantly shallow-water carbonate strata in northern Namibia that was deposited over a 300-km-wide shelf and narrow south-facing foreslope between ~770 and ~590 Ma. Glacial-periglacial deposits of Sturtian (717-660 Ma) and Marinoan (~645-635 Ma) age occur within the carbonate succession. We measured ~250 sections of the synglacial units and their enveloping strata in several continuously-exposed structural panels totalling ~260 km in length. We find that both synglacial units thicken locally to maxima of 1.8 km (Sturtian) and 0.6 km (Marinoan), compared with median thicknesses of 50 and 25 m, respectively. Some thickenings are related to incised paleovalleys, diagnosed by reciprocal relations with underlying strata and/or by depositional onlap. Others represent giant composite moraines and have reciprocal relations with overlying strata only. A Sturtian half-graben at Toekoms is filled by up to 1.8 km of synrift growth strata, mostly laminated siltstone with turbidites and ice-rafted lonestones, plus scarpward-thickening conglomerates and glacial diamictites. A symmetrical U-shaped paleovalley at Omutirapo is filled axially by 510 m of Sturtian glacial diamictite and subordinate sorted deposits. The half-graben hosted a subglacial lake or fjord, while the U-shaped basin was likely excavated by an ice stream. Composite moraines with synoptic heights of 0.22 (Sturtian) and 0.6 km (Marinoan) are remarkably steep, with aspect ratios of 0.15 and 0.08, respectively, in the lines of section. No incision of underlying strata accompanies the Sturtian moraine at Vrede. The Marinoan moraine at Duurwater is situated on the proximal foreslope. Ice grounding lines are stabilized by steep seaward-dipping slopes. Compared with younger examples, Cryogenian moraines more often formed on continental slopes because of larger glacioeustatic falls. Giant composite moraines should accordingly have been more numerous. The power of closely-spaced measured sections to resolve synglacial paleotopography leaves us skeptical of regional sequence-stratigraphic interpretations of Cryogenian glacial deposits based on small numbers of widely-spaced measured sections.