An eolian dust origin for clastic fines of Devono-Mississippian mudrocks of the greater North American midcontinent—Discussion Open Access

In a recently published paper, McGlannan et al. (2022) posit that the detrital silt component of Upper Devonian and Lower to Middle Mississippian shales of the North American midcontinent region is of eolian origin and provided essential nutrients that stimulated the organic productivity that is recorded by these shales. Our initial concern with their study was the fact that McGlannan et al. (2022) studied a stratigraphic interval that spans approximately 20 million years (Late Devonian to Early Mississippian), and applied conclusions reached for the Mississippian part of the succession to the underlying Devonian black shales. This is surprising because it is well known that a drastic sea-level fall occurred at the end of the Devonian and resulted in an unconformity over much of the North American craton (Frazier and Schwimmer 1987; Over 2021). As far as we understand from geologic literature, extrapolating the inferred sedimentary dynamics of one stratigraphic interval (Early Mississippian) across a sequence boundary to rocks that were deposited multiple millions of years earlier (Late Devonian) is neither recommended nor considered good practice.

The aim of this discussion is not to disqualify eolian systems in general as a potential source of fines in shale successions. It is not implausible that there were times and places in the geologic past when they may have contributed to marine or lacustrine shale–mudstone successions. We are, however, firmly convinced that in the case of the Late Devonian of North America, no compelling case can be made for a significant eolian clastic component to its widely distributed black-shale successions.

Whereas we disagree with the validity of the McGlannan et al. (2022) study at several levels, three principal lines of reasoning stand out to demonstrate that their conclusions are flawed and inapplicable to the Late Devonian black shales of North America.

First, petrographic examination of the rocks in question, collected from the same location as samples used by the authors (Woodford samples from McAlister Quarry), as well as samples from the Woodford of the Central Basin Platform (Permian Basin, Texas, samples courtesy of Kitty Milliken) does not support their assumptions about these rocks. SEM analysis shows these samples to be strongly dominated by microcrystalline quartz (Fig. 1) and indicates that it is highly unlikely that the authors could have succeeded to extract detrital quartz grains from these rocks with the method described (McGlannan et al. 2022), much less using them for grain-size analysis. The abundant early diagenetic silica cementation that fed on dissolution of radiolarian tests is so ubiquitous in these shales, generating silt- and even sand-size quartz particles (Blatt 1987; Schieber 1996; Schieber et al. 2000) that upon crushing/processing it would inevitably have generated silt- and sand-size quartz particles (Fig. 2). Petrographic constraints (Fig. 1), however, should readily disabuse a discerning observer from the notion that these particles are in any kind or form detrital in nature.

In our extensive experience with mudstone petrography, the detrital vs. authigenic nature of silt-size quartz grains cannot be determined (as the authors did) via inspection of “smear slides of residues” with a petrographic microscope. To make that determination requires the resolution offered by scanning electron microscopes (SEM). Figure 3 shows our Woodford samples from the perspective of scanned cathodoluminescence, illustrating the type and size range of detrital quartz. The dominant reddish-orange colors of quartz grains suggest derivation from the same low-grade metamorphic source, the Acadian orogeny, presumed for Late Devonian black shales farther east (Schieber 2016). The general abundance and size range of detrital quartz from the Woodford is in essence the same as that observed in Upper Devonian black shales of the Illinois Basin ca. 500 km to the east, and in the Williston Basin ca. 900 km to the north (Fig. 3), plausibly suggesting that all three locales received clastics from the Acadian orogen in the east (Schieber 2016). The lacking differences in size range and source characteristics of detrital quartz (Fig. 3) between samples from Oklahoma and those of time-equivalent Devonian black shales from the Illinois and Williston basins (both far offshore the Acadian source region) supports strong dilution of clastics by early diagenetic silica in the case of the Woodford samples. No principal difference is observed between these locales. The randomly oriented phyllosilicate platelets (cardhouse fabric) observed in these rocks (Figs. 1, 4), shielded from compaction by early diagenetic silica cements (Figs. 1, 2), strongly suggests that the detrital component of these shales arrived via bedload transport of flocculated muds, and is quite similar to uncompacted fabrics seen in flume experiments that simulated accumulation of muds from bottom current transported floccules (Schieber et al. 2007; Schieber 2011). Settling of airborne dust through the water column should produce an entirely different fabric with largely bedding-aligned phyllosilicates (O’Brien and Slatt 1990).

Second, when examined in detail, stratigraphic relationships in Devonian organic-rich mudstone successions show rapid shifts of depocenters in response to basin dynamics (e.g., basement fault reactivation; Jacobi and Fountain 2002; Wilson et al. 2022) as well as accommodation change due to subsidence (tectonic) and sea-level fluctuations. In the Late Devonian, regional unconformities (sequence boundaries) and their correlative conformities have been recognized and correlated across the United States by various authors (Brett et al. 2011; Lazar and Schieber 2022; Wilson et al. 2022). Yet, these erosional contacts (and implicit hiatuses) are absent from Figure 2 of McGlannan et al. (2022), greatly oversimplifying the existing stratigraphic complexities. Stratigraphic analysis shows that in the Appalachian Basin (Smith et al. 2019; Wilson et al. 2022), the Illinois Basin (Lazar and Schieber 2022), and in central Oklahoma (Infante Paez et al. 2017), Devonian black shale successions invariably onlap onto structural highs (Cincinnati Arch, Nemaha Ridge). Such distal responses to shoreline progradation and retrogradation imply lateral sediment supply (for example by wind-driven bottom-current systems) from a distal source and require shoreline-attached sediment dispersal systems. Had eolian sediment dispersal indeed been the key control in the distal realm, stratal units should be expected to drape across positive elements, rather than terminate against them.

The most pervasive chert deposits associated with Late Devonian black shales occur in southern Laurentia, where a proximity to deep ocean waters and a combination of SE Tradewinds with the Ekman spiral provide ideal conditions for upwelling of nutrient-rich waters onto the flooded North American craton (e.g., Parrish 1982; Schopf 1983; Parrish and Barron 1986; Comer 1991). Whereas the influence of this nutrient source likely diminishes away from the cratonic edge (Murphy et al. 2000a, 2000b), it is a highly plausible scenario for driving marine productivity in the Anadarko–Arkoma and Permian basins and the Woodford Shale sections sampled by McGlannan et al. (2022).

Third, due to the Devonian-age colonization of land masses by trees and other land plants, nutrient supply to shelf and epicontinental seas increased so dramatically (Algeo et al. 1995; Algeo and Scheckler 1998) that eolian sourced nutrients, had they existed, would not have made any difference with regard to the available nutrient supply. Also, as mentioned above, the very likely existence of upwelling of nutrient-rich deep-sea waters along the southern margin of the Late Devonian inland sea is a much more plausible nutrient source (rather than eolian input) for that part of the Late Devonian black-shale system. Fundamentally, there is no need, nor any justification, to call on input of eolian clastics as a critical factor for the formation of any of the Late Devonian black-shale successions in North America. Several studies of geochemical proxies and organic petrography have shown marine–terrestrial co-dependencies linking rapid expansion of land plants, enhanced continental weathering, terrestrial runoff, and nutrient supply from the Acadian borderlands to enhanced primary productivity in the Devonian inland sea (Maynard 1981; Algeo et al. 1995; Algeo and Scheckler 1998; Berner 2005; Wilson and Schieber 2017; Song et al. 2021). Collectively, these studies evoke a mental image of coastal plains with thriving forests, unlikely to have facilitated widespread deflation and lofting of siliciclastic fines as postulated by McGlannan et al. (2022). Furthermore, generating copious quantities of dust to fertilize the Devonian inland sea for several millions of years by way of deflation implies a commensurate concentration of sand and formation of eolian dunes (Kocurek 1996). Yet, in spite of a long history of research into Devonian nearshore and onshore deposits of the Appalachian Basin (e.g., Bridge and Willis 1991, 1994; Walker and Harms 1971; Slingerland and Loule 1988) no eolian deposits have ever been documented.

In summary, the confluence of petrographic constraints, stratigraphic relationships, and global controls thoroughly invalidates the premise of McGlannan et al. (2022). The considered evidence shows a striking mismatch between well documented geologic realities and the “eolian supply” vision proposed by McGlannan et al. (2022).