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

Abstract

We thank Wilson and Schieber for their discussion, as our paper in Current Ripples presented a new hypothesis, and we welcome tests of that hypothesis. Current Ripples encourages “provocative papers on sedimentary geology” so we are happy to motivate future research toward advancing knowledge on the Devono-Mississippian of North America.Through an integration of paleogeography, paleoclimate, grain size, detrital-zircon provenance, geochemistry, and surface-wind models, McGlannan et al. (2022) proposed that eolian transport supplied significant siliciclastic material to Devono-Mississippian marine strata of the North American midcontinent. Wilson and Schieber (2024) begin their discussion with the statement that “…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.” We find this odd, akin to arguing that, e.g., sediment dynamics of glacioeustasy in the Pleistocene cannot apply to glacioeustasy that operated in the Pennsylvanian. Processes can apply across time, as long as the tenets of uniformity of process (uniformitarianism) are followed. Wilson and Schieber (2024) then focus on three main arguments to challenge the validity of our hypothesis for the Late Devonian in particular.Regarding the issue of an authigenic or detrital origin for the silica, we recognize that Schieber and his collaborators have extensive experience with mudstone petrography and petrology, are aware of and respect their work documenting diagenetic silica in mudstones, and indeed acknowledge in our paper the pervasive presence of diagenetic and biogenic silica in the Woodford Shale. For example, Figure 5B in McGlannan et al. (2022) illustrates the rhythmic, thin, chert-like beds in the Woodford Shale. Wilson and Schieber note that they have studied samples from the same sites we studied and found diagenetic silica. We do not doubt this. Owing to the common presence of diagenetic silica, we preferentially avoided silica-rich facies and predominantly sampled laminated shale facies. Wilson and Schieber (2024) suggest that we generated sand- and silt-size particles “upon crushing–processing” but, as we detailed in our paper, “Samples were gently crushed with a ceramic mortar and pestle to pea-size gravel to accelerate chemical reactions, then rinsed with distilled water and sieved at 250 μm to remove any fines generated during crushing” that might be erroneously incorporated in the grain-size analyses. Of 19 Woodford Shale samples, 12 were selected for particle-size measurement after smear-slide analysis to verify disaggregation and presence of a preponderance of detrital material (Supplemental File 3 and Fig. 6A in McGlannan et al. 2022), which included not only quartz, but minor feldspar and even (rare) accessory minerals such as zircon. We acknowledge that, despite our efforts to minimize the inclusion of diagenetic silica, some microcrystallized silica may have remained, which would have skewed particle-size results to finer modes. Additionally, as discussed in our paper, eolian delivery of silica-rich dust likely provided a significant source of Si for biogenic and diagenetic silica in these Devono-Mississippian units—an interpretation that builds on previous work by others, such as Banks (1970), Cecil (2015), and Cecil et al. (2018).Wilson and Schieber (2024) then address stratigraphic relationships, implying that we believe the Woodford (and correlative black shales) should exhibit a draping geometry owing to hemipelagic settling, but we do not, in any way, dispute a component of bottom-current transport for the redistribution of detrital material in the Woodford Shale or any of its correlative formations. We absolutely acknowledge the action of submarine redeposition and redistribution. We mention several times throughout the paper that these sediments were ultimately deposited in the marine environment, but we hypothesize eolian delivery of the material to the marine system. Once eolian dust hits the surface waters, it is subject to marine processes, especially if accumulation occurs on submarine slopes. Analogous systems wherein submarine mass flows redistributed eolian-transported fines include the sandstones and siltstones of the Permian Delaware Mountain Group, for example (Fischer and Sarnthein 1988).We do not doubt the existence of stratigraphic complexities related to basin dynamics and sea-level fluctuations. Stratigraphic complexities in the Woodford Shale related to relative changes in base level are certainly present in Oklahoma, but the comparison of these complexities to those in the Appalachian foreland basin and the intracratonic Illinois and Michigan basins requires much further stratigraphic study. During the Late Devonian the Anadarko basin was essentially a wide passive margin facing southward into the Rheic Sea. Although there appears to be an intra-Woodford unconformity at the Devonian–Carboniferous boundary, the continuity in conodont and pollen biozones at the I-35 South outcrop in Oklahoma (Over 1990, 1992; Kondas et al. 2018) and conodont zones at the Wapanucka (Over 1990), and Hass G (Boardman and Puckette 2006; Haywa-Branch and Barrick 1990; Over and Barrick 1990) locations demonstrate that the Upper Woodford unconformity is of a lower magnitude than certain regions in the up-dip interior Illinois basin (Over et al. 2019; Over 2021; Fig. 6 in Lazar and Schieber 2022). Yet, the sediments are similar; therefore eolian delivery provides a viable means for transporting sediments beyond the Appalachian system.In their third major point, Wilson and Schieber (2024) take issue with the hypothesis of eolian nutrient fertilization. Regarding nutrients supplied to Devonian epeiric seas, we accept the possibility of eutrophication from the expansion of terrestrial land plants, and deep basin upwelling, in addition to eolian dust fertilization. We did not exclude any mechanism by which nutrients might have reached the marine system. Rather, we merely suggested dust as a possible carrier of iron, in light of the eolian hypothesis. We are raising questions and proposing hypotheses to consider. We do not know how significant dust may (or may not) have been in delivering nutrients (bioavailable iron) across Laurentian epeiric seas during the Late Devonian. Mechanisms of nutrient delivery are not mutually exclusive. In other words, sometimes the answer isn’t either/or, but both, or all the above.Finally, we disagree with Wilson and Schieber’s (2024) argument on the requirement for “a commensurate concentration of sand and formation of eolian dunes” to generate “copious quantities of dust.” As addressed in our paper, whilst much controversy surrounds the claim that sand dunes and eolian processes produce significant dust deposits, both empirical and experimental studies raise questions about the efficacy of sand saltation in producing silt, and this remains a contentious claim. For example, although sand saltation has been suggested for small loess accumulations such as the Negev in Israel (Crouvi et al. 2008), relatively minimal eolian silt (loess) occurs in the peri-Saharan region, for example (Smalley 1995)—where one might expect large volumes given the vast sand seas. Furthermore, recent experimental studies demonstrate the inefficiency of eolian saltation of sand for silt production (Swet et al. 2019, 2020; Adams and Soreghan 2020). Consider that the dustiest source on the planet today, the Bodélé Depression, is not sourced by dunes, but by the desiccated-lake deposits of paleolake Megachad (Warren et al. 2007). Similarly, dust off the Copper River Delta today is sourced by the deflation of glacially generated fines across (dried) floodplains, not sand dunes (Crusius et al. 2011).We again thank Wilson and Schieber for the opportunity for additional discussion, and for highlighting the need for further petrographic and SEM studies of the Woodford Shale focused specifically on the detrital component. It is likely we will continue to agree to disagree on several points; even so, we look forward to further testing and revision of our hypothesis that eolian-transported detrital silt and dust contributed to the Woodford Shale, and many of its correlatives.