'Zipper-Rift': a Tectonic Model for Neoproterozoic Glaciations During
Total Page：16
File Type：pdf, Size：1020Kb
Earth-Science Reviews 65 (2004) 1–73 www.elsevier.com/locate/earscirev ‘Zipper-rift’: a tectonic model for Neoproterozoic glaciations during the breakup of Rodinia after 750 Ma Nicholas Eyles*, Nicole Januszczak Department of Geology, Environmental Earth Sciences, University of Toronto at Scarborough, 1265 Military Trail, Scarborough, ON, Canada M1C 1A4 Received 9 October 2002; accepted 28 May 2003 Abstract The ‘Snowball Earth’ model of Hoffman et al. [Science 281 (1998) 1342] has stimulated renewed interest in the causes of glaciation in Earth history and the sedimentary, stratigraphic and geochemical response. The model invokes catastrophic global Neoproterozoic refrigerations when oceans froze, ice sheets covered the tropics and global temperatures plummeted to À 50 jC. Each event is argued to be recorded by tillites and have lasted up to 10 million years. Planetary biological activity was arrested only to resume in the aftermath of abrupt and brutal volcanically generated ‘greenhouse’ deglaciations when global temperatures reached + 50 jC. The ‘Cambrian explosion’ is regarded by some as a consequence of post-Snowball glacioeustatic flooding of continental shelves. We shall show by a systematic review of the model that it is based on many long standing assumptions of the character and origin of the Neoproterozoic glacial record, in particular, ‘tillites’, that are no longer valid. This paper focusses on the sedimentological and stratigraphic evidence for glaciation in the light of current knowledge of glacial depositional systems. By integrating this analysis with recent understanding of the tectonic setting of Neoproterozoic sedimentary basins, an alternative ‘Zipper-rift’ hypothesis for Neoproterozoic glaciations is developed. The ‘Zipper-rift’ model emphasises the strong linkage between the first-order reorganisation of the Earth’s surface created by diachronous rifting of the supercontinent Rodinia, the climatic effects of uplifted rift flanks and the resulting sedimentary record deposited in newly formed rift basins. Initial fragmentation of Rodinia commenced after about 750 Ma (when the paleo-Pacific Ocean started to form along the western margin of Laurentia) and culminated sometime after 610 Ma (with the opening of the paleo-Atlantic Ocean on the eastern margin of Laurentia). Breakup is recorded by well-defined ‘tectonostratigraphic’ successions that were deposited in marine rift basins. The base of each succession is characterised by coarse-grained synrift strata consisting of mass flow diamictites and conglomerates (many of the ‘tillites’ of the older literature). These facies are interbedded with large olistostromes and contain clastic carbonate debris derived from landsliding of fault scarps along rifted carbonate platforms. Diamictites and conglomerates are dominantly the product of subaqueous mass flow and mixing of coarse and fine sediment populations (the term mixtite has been used in the past). These facies are not uniquely glacial and are produced regardless of climate and latitude. Synrift deposits commonly pass up into thick slope turbidites recording enhanced subsidence and are capped by uppermost shallow marine strata that record a reduction in subsidence rates and overall basin shallowing. Tectonostratigraphic ‘cycles’ can attain thicknesses of several kilometres, but have been commonly misinterpreted as recording globally synchronous ‘glacioeustatic’, falls and rises in sea level. In fact, eustatic sea-level changes in rift basins are * Corresponding author. Tel.: +1-416-287-7231; fax: +1-416-287-7279. E-mail address: [email protected] (N. Eyles). 0012-8252/$ - see front matter D 2003 Elsevier B.V. All rights reserved. doi:10.1016/S0012-8252(03)00080-1 2 N. Eyles, N. Januszczak / Earth-Science Reviews 65 (2004) 1–73 suppressed as a result of a strong tectonic control on relative water depths. The great length of newly formed rifted margins around the long perimeter of Laurentia ( < 20,000 km) ensured that deposition of tectonostratigraphic successions occurred diachronously as in the manner of a zipper between approximately 740 and 610 Ma. Some successions show a definite glacial influence on sedimentation as a consequence of latitude or strong tectonic uplift, but many do not. Regardless, all deposits record a fully functioning hydrological cycle entirely at odds with a supposed fully permafrozen planet. Mapping of those deposits where a definite glacial imprint is apparent indicates that Neoproterozoic glaciation(s) were likely regional or hemispheric in scope and latitudinally constrained. They were perhaps no more severe than other glaciations recorded in Earth history. The regional distribution of ice centres is argued to have been influenced by tectonic topography created by large mantle plumes and by rift shoulder uplift. Paleomagnetic data indicative of tropical glaciation are, in our view, ambiguous because of uncertainty as to when such paleomagnetic characteristics were acquired. A lower solar luminosity may have played a role in lowering snow line elevations and displacing glaciation into latitudes lower than those of Phanerozoic glaciations. Global tectonic and volcanic activity, especially the rapid burial or organic carbon in new rift basins, may explain extreme shifts in C-isotopic values evident in late Neoproterozoic strata. D 2003 Elsevier B.V. All rights reserved. Keywords: Neoproterozoic; ‘Zipper-rift’; Snowball Earth; Rodinia; Rifting; Tectonic topography; Glaciation 1. Introduction data with information regarding Neoproterozoic tec- tonic settings, a new model for Neoproterozoic glaci- Poorly sorted Neoproterozoic diamictites, inferred ations is proposed. We argue that Neoproterozoic by many geologists to be direct sedimentary products glaciations were regional in extent and probably no of large continental ice sheets (tillites), are found on different in character and thermal regime from those all continents and were deposited between approxi- of the Phanerozoic. Mapping of glacial strata across mately 750 and 610 Ma. In the absence of site- Rodinia indicates that a key factor allowing the specific age data on key stratigraphic horizons, Neo- growth of ice covers may have been extensive uplift proterozoic diamictites are still viewed by many as of rift shoulders and plateaux in mid-to-high latitudes precise age-correlative marker horizons recording during the protracted rifting of the supercontinent globally synchronous changes in climate and eustatic Rodinia. Rifting began after 750 Ma as Laurentia sea level. More recently, it has been argued, in progressively ‘unzipped’ from Rodinia to form the conjunction with rapid changes in 13C and 87Sr/86Sr paleo-Pacific Ocean. A second phase of rifting and ratios, that the planet was repeatedly and entirely ice- glaciation occurs at about 610 Ma when Baltica broke bound for many million years when nearly all organic away from the eastern margin of Laurentia to form the activity ceased and oceans froze to depths of 1 km or Iapetus Ocean (paleo-Atlantic). more under global temperatures of À 50 jC(the ‘‘Snowball Earth’’). Evidence for the Snowball Earth model focusses 2. The paradox of Neoproterozoic climate; on geochemical and geophysical (paleomagnetic) catastrophic change or business as usual? data along with the presence of so-called ‘cap carbo- nates’ argued to record rapid global deglaciation. In Neoproterozoic successions containing diamictite contrast, the tectonic setting, sedimentology and facies, often of great thickness, occur on every depositional environment of supposed glacial depos- continent. Currently, this record is interpreted by its have been largely ignored by proponents of the different groups of geoscientists either as a series Snowball model. of catastrophic changes without parallel in Earth Our intent here is to refocus debate regarding history (Williams, 1994; Hoffman and Schrag, Neproterozoic climates on the sedimentologic and 2002; Hoffman et al., 1998a) and having fundamen- stratigraphic record of glaciation. By scrutinizing this tal implications for the biological evolution on the record in the light of recently gained knowledge of planet (‘‘a script for global catastrophe’’, Runnegar, glacial depositional systems and by combining such 2000) or as being a record of glaciation little differ- N. Eyles, N. Januszczak / Earth-Science Reviews 65 (2004) 1–73 3 ent from that of other glacial epochs which were 2.1. Diamictites; glacially deposited tills, glacially controlled primarily by tectonics and geography influenced marine or nonglacial? (Eyles, 1993; Crowell, 1999; Leather et al., 2002). At the heart of the debate lie very different interpre- Diamictites (lithified diamicts) are poorly sorted tations of the origin and paleoclimatic significance of ‘concretelike’ facies and are a highly conspicuous diamictite facies. component of many Neoproterozic stratigraphic suc- Fig. 1. Glacial and nonglacial environments in which diamict facies are produced. 4 N. Eyles, N. Januszczak / Earth-Science Reviews 65 (2004) 1–73 cessions. They are often labelled as ‘glacial’ simply on Neoproterozoic diamictites and associated facies that the basis of their poor textural sorting and physical are no longer compatible with modern knowledge of resemblance to the deposits left behind by modern glacial depositional systems. wet-based glaciers. These glaciers slide over their In Part II of this paper, we review the global beds and generate coarse-grained, poorly sorted glaci- sedimentologic and stratigraphic record of Neoproter-