A sedimentary model for early Palaeozoic fluvial fans, Alderney Sandstone Formation (Channel Islands, UK)
Introduction
The prominence of fluvial systems with distributary planform (e.g., alluvial and fluvial fans) in the rock record is the subject of heated debate (Nichols and Fisher, 2007, North and Warwick, 2007, Weissmann et al., 2010, Fielding et al., 2012), and recent reviews concurred that more field-based evidence is required, particularly to test the models for large fluvial fans (Latrubesse, 2015, Weissnann et al., 2015). It is accepted that down-fan discharge partitioning along bifurcating channels produces distinctive trends, such as grain-size fining, channel-body shrinking, reduced channel-body interconnectivity, apparent shift towards more seasonal discharge, and change in palaeosol draining (Friend, 1978, DeCelles and Cavazza, 1999, Horton and DeCelles, 2001, Cain and Mountney, 2009, Ralph and Hesse, 2010, Weissmann et al., 2013, Owen et al., 2015a). These trends are well perceived in vegetated (i.e., late Palaeozoic and younger) fluvial systems characterised by well-defined channelization, significant amounts of mud in their floodplains, and well-developed palaesols (Davies and Gibling, 2010). By comparison, the recognition of some of these trends in pre-Silurian fluvial rocks is challenged by the mud-free nature of many terrestrial deposits (Went, 2005, Marconato et al., 2014), by the scarcity of channel forms at the outcrop scale (Eriksson et al., 1998, Long, 1978, Long, 2011), and by the almost complete lack of pedogenic features related to biotic processes (Álvaro et al., 2003). These features have been ascribed to dominant physical over chemical alteration (Went, 2005, Davies et al., 2011), and lack of sediment binding by plants alongside mobile fluvial channels (Long, 2006, Long, 2011). For these reasons, it still has to be tested whether the sedimentary models developed for post-Silurian fluvial fans apply to pre-vegetation ones. A number of Precambrian and early Palaeozoic terrestrial deposits have been interpreted as proximal distributive systems (i.e. alluvial fans; Williams, 1966, Williams, 2001, Went, 2005, Chakraborty et al., 2009, Chakraborty and Paul, 2014, Santos et al., 2014), while the proper recognition of larger and more mature fluvial fans remains limited (Eriksson and Vos, 1979, Eriksson et al., 2006).
The main goal of this research is to discuss a set of criteria for the detection of fluvial fans in pre-vegetation environments. These criteria are based on sedimentologic and depositional-architectural analysis, at both outcrop and remote-sensing scales, of the Cambrian Alderney Sandstone Formation of Channel Islands, UK (Fig. 1A). The Alderney Sandstone Formation is regarded as a fine example of early Palaeozoic fluvial sedimentation (Todd and Went, 1991, Davies et al., 2011). Deposited in proximity to basement highlands, and confined within a post-orogenic basin of ~ 200 × 100 km, the formation is currently believed to represent a set of axial fluvial trunks sided by basin-margin systems (Went and Andrews, 1990, Davies et al., 2011). This work provides a partial reappraisal of the Alderney Sandstone Formation, emphasizing the volumetric importance of fluvial-fan deposits and describing aeolian facies not documented to date. This study demonstrates that pre-vegetation fluvial fans, despite lacking heterolithic alluvium and well-developed channelization, can be differentiated from axial fluvial trunks based on the distinctive up-section trends in degree of fluvial-aeolian interaction, fluvial-bar shrinking due to drainage fractionation, and increased dispersion of cross-set thickness values. Given the nature of the studied exposure (a gently folded monocline; Fig. 1), this study is particularly intended to contribute to the recognition of fluvial fans based solely on lateral and vertical facies trends, i.e. in those situations where inferences on basin-scale planform cannot be directly drawn (cf. Trendell et al., 2013, Owen et al., 2015b). Finally, although any analogy to modern or extra-terrestrial fluvial systems is challenging, potential parallels with modern fluvial-aeolian landscapes are explored, providing two partial analogues located in the deserts of Mexico and Turkmenistan.
Section snippets
Geological setting
A suite of lower Palaeozoic clastic successions are exposed at several locations in the Channel Islands (UK), and Brittany and Normandy (Northern France), attaining a maximum thickness of 800 m (Sutton and Watson, 1970, Pudsey, 1977, Laffoley, 1986, Went and Andrews, 1990, Doré, 1994). Regionally, these strata record depositional environments that include proximal alluvial fans and mature fluvial systems passing basinward into nearshore-marine realms (Todd and Went, 1991, Went, 2005, Went, 2013
Methods
Field methods consisted of bed-by-bed logging, which assisted analyses of sedimentology and depositional architecture. Stratigraphic logging was performed with 5-cm vertical resolution following the guidelines provided by Collinson et al. (2006). A total of 881 palaeocurrent indicators were measured based on the azimuth of accretion of cross-beds, and corrected for tectonic tilt. Line-drawing of photographic panels aided in the identification of different architectural elements (cf. Miall, 1994
Description
Fluvial deposits consist of six sedimentary facies, summarized in Table 1. Roughly 80% of the study section is represented by medium- to coarse-grained arkose with subordinate arkosic wackes and intra-formational pebble-conglomerate. The arkose shows poor to moderate sorting (Fig. 3A) and is composed of sub-angular, non-spherical grains (Fig. 3B). Thin-sectioned samples reveal absence of matrix, grain fracturing (Fig. 3C-D), and quartz dissolution, with edges of quartz grains ‘protruding’
Depositional architecture
The analysis of depositional architecture is based on the identification and rudimentary interpretation of bounding surfaces on outcrop and satellite imagery. Master bounding surfaces are the most prominent and consist of extensive erosional surfaces marked by sharp contrasts in erodibility (Fig. 7). Master bounding surfaces have irregular erosional relief with metric scale; exceptionally, up to 4 m of erosional relief are observed over 15 m along strike (Fig. 7C). The contrast in erodibility
Discussion
The Alderney Sandstone Formation was previously related exclusively to deposition by axial-fluvial systems (Went and Andrews, 1990). The occurrence of sandstone foresets systematically showing accretion at ~ 60° with the main fluvial palaeoflow was ascribed to an unilateral fluvial migration (‘combing’), controlled by tectonic tilting of the alluvial plain (Todd and Went, 1991). Until the revised analyses by Chakraborty et al. (2010) and Sinha et al. (2013), the apparent unidirectional shift
Conclusions
The Corblets and Quesnard sandstones, part of Cambrian Alderney Sandstone Formation and previously considered as solely the products of fluvial processes, are reappraised as the record of an aeolian-influenced fluvial fan. At the core of this reinterpretation is the first documentation of a significant volume of aeolianites in the type section of eastern Alderney Island (UK). Field and remotely sensed data depicted the association of three types of architectural elements (compound fluvial bars,
Acknowledgments
We thank the insightful comments of reviewer André Marconato (University of São Paulo) and Editor Brian Jones. Dario Ventra is also thanked for stimulating discussions. AI was in part supported by a post-doctoral fellowship granted by the Natural Sciences and Engineering Research Council of Canada.
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