Alluvial facies evolution during the Palaeozoic greening of the continents: case studies, conceptual models and modern analogues

Abstract

The Palaeozoic greening of the continents -the appearance and expansion of embryophytes (land plants) in terrestrial environments -was arguably the most fundamental Phanerozoic change to the Earth system. Thirteen case studies of Cambrian to Devonian fluvial deposits from North America and Europe are documented here to illustrate the evolution of fluvial style during this period. During the Cambro-Ordovician, prior to the advent of terrestrial vegetation, fluvial systems laid down relatively coarse sands with little mud, resulting in self-formed channels and an architecture dominated by broad sheets of trough cross-beds (sheet-braided style). Similar deposits formed across a wide range of latitudes, and passed basinward into sandy coastal deposits. From the mid Ordovician onwards, an increase in floodplain mudstone corresponds broadly with the appearance of embryophytes, which would have progressively enhanced upland weathering, mud production and floodplain storage of fines. During the Late Silurian, small heterolithic channel bodies with lateral-accretion sets provide the first evidence of meandering channels, and floodplain mudstones contain more varied palaeosols, especially calcretes which appear abundantly for the first time. By the Early Devonian, channel deposits comprise sandstone lenses (channelled-braided style), probably due to the increased bank strength and cohesion and reduced potential for sediment sorting imparted by sand-mud mixtures. Muddy coastal deposits are prominent. By the Upper Devonian, fluvial deposits commonly contain fossil trees and large mainstem meandering channels with lateralaccretion sets, indicating that rooted vegetation stabilized channels. The advent of stable floodplains with levées, crevasse splays and organic litter would have encouraged the diversification of terrestrial invertebrates, which left alluvial ichnological signatures from the Late Silurian onwards.

Early Cambrian metazoans in fluvial environments, evidence of the non-marine Cambrian radiation: COMMENT

“…It is acutely inadvisable to use the absence of such physical features as support for a fl uvial origin, as advocated by Kennedy and Droser. Other evidence used by Kennedy and Droser to identify the middle Wood Canyon as fl uvial largely originates from the conclusions of previous studies. However, in a more recent fi eld study we suggested that the presence of trace fossils implies the middle Wood Canyon was deposited within a distal, marine-infl uenced braid plain (Davies et al, 2011b). All the sedimentary characteristics listed by Kennedy and Droser could have been formed in a marine-infl uenced setting.…”

“…Neither we nor they found trace fossils in any of these, which in some cases constitute hundreds of meters of well-exposed strata (Davies and Gibling, 2010;Davies et al, 2011b). Studies suggest that pre-vegetation alluvial systems were highly unstable landscapes, where overbank deposits were transient accumulations prone to erosion by channel migration and eolian winnowing, and where rivers themselves were sheets of barren sand prone to fl ashy ephemeral fl ooding events.…”

“…The physical and ecological marine-fl uvial interfaces in modern environments are spatio-temporally gradational and fl uctuant (Dalrymple and Choi, 2007). During the early Paleozoic, when fl uvial activity lacked the regulating infl uence of vegetation, such interfaces were even more prone to back-and-forth migration due to the balance of episodes of fl ashy progradational sedimentation and subsequent marine fl ooding (Cotter, 1983;Davies et al, 2011b). In such settings, particularly given the shallow cratonic gradient of many Cambrian continental-marine transects, marine conditions may have extended far landward during periods of fl uvial quiescence, permitting the overprinting of alluvium with a marine ichnofabric and accessory mudrocks.…”

Phanerozoic environments of black shale deposition and the Wilson Cycle

“…Whereas many depositional environments are nonuniformitarian because they are absent in the presentday tectonic configuration, exogenic processes -at least in the Phanerozoic -are in all likelihood uniformitarian, although certain evolutionary trends (cf. Davies et al, 2011) have led to changes in the response to these processes.…”

“…Late Devonian algal-rich black shales, which were deposited at high southern latitudes along the western margin of Gondwana (Brazil and Bolivia) under an upwelling zone, are a further example of black shale deposition on the margins of a mature ocean basin. It should be noted that, in both examples, land plants had not yet or were only poorly developed at that time (Davies et al, 2011).…”