A Neoproterozoic Rock Sample Suite: Evidence for the Snowball Earth Timothy Fox¹, Eugene Domack¹, and Paul Hoffman 2 [email protected], [email protected] Hamilton College, 198 College Hill Road, Clinton NY 13323 [email protected] Geological Museum Rm 203D , 24 Oxford St., Cambridge, MA 02138 Table of Contents The Namibian samples-thumbnail descriptions 2 Namibia TS-1 3 What are Cap Carbonates? 4 Namibia TS-3 5 Namibia TS-12 6 Namibia TS-15 7 Namibia AB-1 8 Namibia TS-20 9 Namibia TS-7 11 The East Greenland samples-thumbnail descriptions 12 East Greenland #36 13 East Greenland #22 14 East Greenland #17 15 East Greenland #7 16 East Greenland #38 17 East Greenland #29 18 East Greenland #50 19 East Greenland D-6 20 Unique mineral fans from Canada’s Mackenzie Mtns. & W. Africa 21 -thumbnail descriptions Northwest Canada PFH-4 22 Northwest Canada P10A 23 West Africa PFH-1 & PFH-2 24 Northwest Canada PFH-3 25 1 0 Sample Characteristics Depositional Context Lwr. TS-1 Marine “cap” carbonate Maiberg deposited directly Fine-grained above the Ghaub 5 carbonate laminite. glacial formation, suggests rapid environmental change. 10 TS-3 Stratified carbonate mud with sediment Represents a major gravity (debris flow) deglaciation event at 15 deposits and the end of the global Section 2-West significant ice rafted Marinoan glacial. debris. 20 TS-12 Stratified carbonate Interbedded diamictite 40 mud diamictite with and sediment gravity sediment gravity debris flows, deposited debris flow deposits as ice receded from the and ice rafted debris. continental shelf. UPPER GHAUB 45 TS-15 50 Massive clast-rich Basal tillite deposited meters 0 diamictite. in direct contact with glacial ice. 5 TS-7 Stratified carbonate Implies slope instability diamictite with and intense ice-rafting 10 sediment gravity flow in response to glacial and ice rafted advance. LOWER GHAUB debris. 15 TS-20 Section 2-East Records the onset of Carbonate mudstone the Marinoan glaciation 20 with calcitic represented by dropstone. dropstones in the Ghaub formation. 25 AB-1 “Cap” carbonate from Fine grained the sequence directly carbonate laminite overlying the older 30 (lithologically similar Cryogenian glacial Upr. to TS-1 of the Lower event, represented by Ombaatjie Maiberg formation). the Rhastoff formation . 2 Namibia TS-1 Kielberg Member of the Maiberg ‘cap’ carbonate succession What are Cap Carbonates? (Taken from www.SnowballEarth.org) Cap carbonates are continuous layers of lime- stone (CaCO3) and/or dolostone (Ca0.55Mg0.45CO3) that sharply overlie Neoproterozoic glacial deposits, or sub-glacial erosion surfaces where the glacial deposits are absent. They are typically 3-30 m thick and occur on platforms, shelves and slopes world-wide, even in regions otherwise lacking carbonate strata. Sturtian 1 cm (~700 Ma) and Marinoan (635 Ma) cap carbonates are Carbonate laminite bounding the Ghaub glacial units lithologically distinct, and both have unusual traits (e.g., Namibia TS-1 is a thinly bedded to laminated allodapic (turbidite) thick sea-floor cements, giant wave ripples, microbial dolostone that lies in sharp contact with the upper member of the Ghaub. mounds with vertical tubular structure, primary and formation. Namibia TS-1 directly overlies the Ghaub glacial deposits. The early diagenetic barite, BaSO4) that distinguish them sharpness of the contact of the upper member of the Ghaub with the from standard carbonates. Marinoan cap carbonates Kielberg cap carbonate implies a rapid transition between glacial and are transgressive (i.e., inferred water depths increase marine deposition. There is limited evidence for seasonal cyclicity in the with stratigraphic height) and most workers associate Kielberg member, although it's interpretation as being a rhythmite has not been ruled out. them with the flooding of continental shelves and platforms as ice sheets melted. Most Sturtian cap carbonates were not deposited until after post-glacial flooding had taken place. The preservation of cap carbonates and related highstand deposits after isostatic adjustment (or post-glacial “rebound”) implies substantial erosion and/or tectonic subsidence during the glacial period. Post-Marinoan (Ediacaran and Pha- nerozoic) glaciations lack cap carbonates, or they are poorly developed. Sample TS-1 In the Field 3 4 Namibia Sample TS-3 Upper Member of the Ghaub Formation Namibia TS-12 Upper Member of the Ghaub Formation 2.5 cm Stratified waterlain debris flows and diamictite with 5 cm 'cap' carbonate transition Stratified waterlain sediment gravity flows and ice rafted debris Namibia TS-3 represents the transition out of the Ghaub glaciation Namibia TS-12 is composed of interbedded, thinly bedded to and was deposited as a marine-based ice sheet receded from an outer shelf. laminated sandstone, calcareous argillite, and thin diamictite. The It lies stratigraphically above the massive basal tillite of the middle member compositional banding is diffuse and elongate with distinct layering of the Ghaub formation and directly below the Kielberg "cap" carbonate most likely deposited in response to local sediment supply and slope unit. The upper member of the Ghaub formation is associated with fluidized instability. Current ripple cross laminations (reference) in the ferrugine- sediment gravity flows, ice-rafted debris, and a sharp contact with the ous sandstone bedding indicate the direction of transport and deposi- overlying 'cap' carbonate of the Kielberg formation. This sample is primar- tion during the Ghaub glacial recessional event. Periods between ily composed of fine carbonate muds interbedded with diamictite debris sediment gravity flows are marked by fine-grained hemipelagic carbon- flows. Debris flows are dominated by stomatolitic boulders covering all size ate mud layers with ice-rafted debris. ranges. The very thin to thickly laminated dolomitic (ferrugineous) sand- stone shows current ripple cross-lamination, indicating flow direction during the time of deposition. The abrupt transition from glaci-marine turbidity current deposition into the pink "cap" carbonate unit suggests that the transition out of the Ghaub glaciation was rapid. Sample TS-12 in the field Sample TS-3 in the field 5 6 Namibia TS-15 Namibia AB-1 Middle Member of the Ghaub Formation Rhastoff Formation 3 cm Massive Clast rich carbonate basal tillite (diamictite) 2 cm TS-15 represents the middle member of the Ghaub formation, Namibia AB-1 is gray and tan carbonate laminite from the it is a poorly stratified to structureless diamictite, with a mostly gray Rhastoff formation. This is a sample of the cap carbonate (typically limestone matrix. Clasts within the diamictite are unsorted and show deposited in shallow warm marine environments) unit that directly evidence of significant lateral strain. Clasts are primarily composed of overlies the Chuos formation (interpreted as distinctly glacial in origin). carbonate and quartz with evidence for intense silica recrystallization. The Chuos formation represents the deposition that occured TS-15 is interpreted as being a basal tillite (diamictite) com- following the older, Sturtian glaciation around 710 million yrs. ago. The posed of unsorted carbonate debris supported by a calcitic limestone laminations lack any significant evidence of being influence by seasonal matrix that was eroded from the underlying Otavi platform as a result cyclicity and are interpreted as allodapic (turbidite) in origin. of ice motion. Rhastoff Cap Carbonate Sequence, Northwestern Namibia TS-15 in outcrop on the Fransfontein ridge Northern Namibia (Diagram courtesy of www.Snowballearth.org) 7 8 Ice rafting and soft sediment deformation in the lower member of the Ghaub Namibia TS-20 Lower Member of the Ghaub Formation The polished surface of TS-20 reveals deformed bedding layers that suggest soft-sediment deformation occurred at some point after deposition of the lower member of the Ghaub. The excessive weight that would have exerted itself on the underlying sediments during the Ghaub glaciation more than adequately accounts for this deformation. 2 cm Dropstones like the one in TS-20 occur repeatedly through- out the thin stratified unit of the lower member of the Ghaub forma- tion. The lower member makes a gradational transition into the much thicker and more massive unit of unstratified carbonate Stratified waterlain carbonate mudsone with ice-rafted diamictite that makes up the middle member of the Ghaub forma- dropstone tion. The transition between the lower member (clast-poor, strati- fied waterlain diamictite) and middle member (clast rich, massive Namibia TS-20 is composed of finely stratified mudstone basal till) of the Ghaub represent ice sheet advance, and possibly the and ferrugineous dolostone with coarse sand grains scattered onset of a Snowball event. throughout. The fine grained primary bedding lacks any signifi- cant evidence of cyclicity suggesting TS-20 was deposited by turbidity currents in an ice distal environment. The sedimentary bedding bounding the two centimeter wide calcitic clast clearly indicates that this is a dropstone deposited by the rain-out of ice-rafted debris. Evidence for ice rafting is derived from the lateral continuity of layers bounding the calcitic clast; layers directly beneath the clast are deformed and appear impacted by the clast's emplacement, whereas, the layers directly above the clast appear draped and un-deformed-implying that their deposi- tion was after that of the clast. 9 10 Namibia TS-7 Lower Member of the Ghaub Formation 5 cm Stratified