Origin and hydrology of a large, intact Early Cambrian paleocave system and its role in overlying fluidisation structures, Arctic Canada
Introduction
Although numerous modern cave systems in the interiors of continents and on modern carbonate islands exposed well above present-day sea-level consist of large dissolutional subsurface structures such as caves and solution-widened joint networks, ancient karst systems consisting of large caverns are not common, and generally have not remained open or intact. Although the fluid dynamics of groundwater in karst are almost exclusively understood from modern karst systems that are large enough to be visited or tested by humans (e.g., Fleury et al., 2007, Bayari et al., 2011, Baechler and Boehner, 2014, Kogovsek and Petric, 2014), direct evidence of fluid behaviour in ancient karst systems is seldom preserved. Most studies of paleokarst focus on surface structures or petrographic attributes of associated meteoric alteration (e.g., Meyers, 1988, Wright, 1988, George and Powell, 1997, Praekelt et al., 2008).
Enigmatic cylindrical sedimentary structures have been documented in the rock record since Hawley and Hart (1934) described large sandstone cylinders in the Cambrian Potsdam Formation in New York and Ontario (e.g., Gabelman, 1955, Draganits et al., 2003, Van Loon and Maulik, 2011), and have even been documented on Mars (Rubin et al., 2017). These structures, referred to as sand volcanoes (or sand volcano feeder pipes if the surficial cone is absent) have, over the years, been attributed to either the upward expulsion of water and sand (e.g., Gabelman, 1955, Li et al., 1996, Massari et al., 2001) or the downward movement of sand into underlying cavities (e.g., Buck and Goldring, 2003, Sanford and Arnott, 2010). Although experimental studies on the formation of such structures have been conducted (e.g., Nichols et al., 1994, Ross et al., 2011) and modern equivalents have been documented (e.g., Li et al., 1996, Draganits and Janda, 2003) there is no consensus about the mechanism of sand volcano formation in the rock record.
Using a combination of depositional environment, sedimentary architecture, and paleogeomorphology, this entirely field-based study (a) examines and explains an extraordinary system of very large, intact paleokarstic caverns associated with a Precambrian-Cambrian unconformity, (b) scrutinises the groundwater hydraulic system associated with nearby shallow-marine sand volcanoes, and (c) examines the implications of the karst and sand volcanoes for the paleotopography and paleoclimate of low-latitude northwestern Laurentia during Early Cambrian sea-level rise.
Section snippets
Geological background
The rocks described in this study are exposed on remote northwestern Victoria Island (Northwest Territories, Canada) and belong to the middle Neoproterozoic Wynniatt Formation (dolostone; Young, 1981, Thomson et al., 2014) and Early Cambrian Quyuk formation (informal nomenclature; sandstone; formerly map-unit 10a of Thorsteinsson and Tozer, 1962, Dewing et al., 2015, Durbano et al., 2015).
The study area exposes the upper part of the ~ 5-km-thick Neoproterozoic Shaler Supergroup, including the
Methods
This study is entirely based on field data. The site is accessible only by helicopter; the nearest permanent helicopter base is approximately 500 km away. The approximately 1.5 km2 mapped study area near the head of Minto Inlet on western Victoria Island preserves the contact of the Neoproterozoic upper carbonate member of the Wynniatt Formation with the Cambrian Quyuk formation (Fig. 2). A mobile GPS/computer was used to obtain the co-ordinates of sandstone bodies inside the dolostone and of
Results
The unconformity near Minto Inlet is associated with the following features: (1) an unsculpted unconformity surface, generally covered, separating underlying Neoproterozoic dolostone from overlying Cambrian sandstone and dipping slightly to the southeast; (2) two carbonate rock structures 40 m in diameter protruding up 10 m above the average level of the unconformity; (3) a group of large (30–100 m diameter and ~ 5–10 m height), horizontally aligned, sandstone-filled voids approximately 30 to 60 m
Interpretation
Although the Proterozoic – Cambrian unconformity at the study site is in the upper Wynniatt Formation, it reaches stratigraphic units lower in the Shaler Supergroup in the Walker Bay anticline and Holman Island syncline (Fig. 1; Rainbird et al., 2013a, Rainbird et al., 2013b, Rainbird et al., 2014). Regional variation of the stratigraphic position of the Proterozoic – Cambrian unconformity indicates that its development post-dated folding and faulting of the Shaler Supergroup. Because the
Tectonic history
The relative timing of fault activity and unconformity development can be deciphered from the geological map of the area (Rainbird et al., 2013a, Rainbird et al., 2013b, Rainbird et al., 2014). After deposition of the entire Shaler Supergroup in the Neoproterozoic (Fig. 6A), compression resulted in folding, and gentle tilting (~ 10° southeast) (Fig. 6B). The low regional dip and thickness of the Wynniatt Formation upper carbonate member (maximum ~ 400 m; Thomson et al., 2014), is insufficient to
Conclusions
The Precambrian-Cambrian unconformity near the eastern end of Minto Inlet on Victoria Island preserves large, intact, sandstone-filled, karstic paleocaverns of Cambrian age (~ 100 m wide; ~ 10 m high) and gryke development in Neoproterozoic dolostone. Caves were dissolved by high-CO2, atmospherically equilibrated, meteoric water along a water table in a nearshore island just prior to its inundation during the early Cambrian Sauk marine transgression. Sand infiltrated caverns and grykes regionally
Acknowledgements
Fieldwork was funded by the Geological Survey of Canada's Geomapping for Energy and minerals (GEM) Victoria Island project (2010 − 2012). Andrew Durbano assisted in fieldwork and provided constructive discussion. Valuable input regarding sedimentary processes and geologic history were provided by Darrel Long, and Danielle Thomson. Suggestions and input from the reviewers and editor greatly enhanced the quality of the manuscript.
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