https://doi.org/10.1130/G45569.1 Manuscript received 10 September 2018 Revised manuscript received 27 December 2018 Manuscript accepted 28 December 2018 © 2019 Geological Society of America. For permission to copy, contact [email protected]. Published online XX Month 2019 Shallow-water onlap model for the deposition of Devonian black shales in New York, USA Langhorne B. Smith1*, Juergen Schieber2, and Ryan D. Wilson3 1Smith Stratigraphic LLC, 21 Ricks Road, Woodstock, New York 12498, USA 2 Department of Geological Sciences, Indiana University,1001 East 10th Street, Bloomington, Indiana 47405-1405, USA 3Chevron Energy Technology Company, 1500 Louisiana Street, Houston, Texas 77002, USA ABSTRACT northwest by the Findlay-Algonquin arch during Field relations, regional correlations, and sedimentological observations show that the the Middle and Late Devonian. Thrust-loading to organic-rich Middle to Upper Devonian Marcellus to Dunkirk black shales (eastern United the east led to subsidence along the thrust front States), commonly interpreted to have been deposited in deep, quiet, and permanently anoxic to the east and uplift on the Findlay-Algonquin basins, were instead deposited in relatively shallow settings that at times had moving cur- and Cincinnati arches to the west. This produced rents and oxygenated bottom water that supported benthic life. Traditionally interpreted to a major decrease in stratal thickness from east to downlap onto a deep basin floor, regional wireline-log correlations linked to outcrop studies west and the development of unconformities on show that these black shales instead onlap unconformities on a tectonic high to the west. The uplifted tectonic highs (Ettensohn, 1994). The basic elements of the proposed model are present in a range of other occurrences around Devonian Catskill Delta complex is composed of the world, and this shallow onlap model should be considered as a viable hypothesis for the the sediments sourced from the uplifted Acadian origin of any black shale succession. Mountains which include nonmarine sediments and marine sandstones, siltstones, and organic- INTRODUCTION shales in the Devonian Appalachian Basin have poor gray shales to the east and time-equivalent The origin of the Middle and Upper Devo- ranged from 150 m (Kohl et al., 2014) to 200 m organic-rich black shales and limestones farther nian organic-rich black shales of the Appala- (House and Kirchgasser, 1993) to 300 m (Baird to the west (Fig. 1B). There is a vast array of chian Basin, eastern United States, has been and Brett, 1991) and even 900 m (Lundegard names assigned to these units, but to simplify the subject of lively debate for more than a et al., 1985). this for this brief paper, the focus is on the seven century. The preferred interpretation in recent Regional correlations of the black shales in prominent organic-rich black shales (Fig. 1B). decades has been that the organic-rich shales New York show very similar field relations to From oldest to youngest these are the Marcellus, were deposited in a deep, quiet, anoxic basin. those found by Conant and Swanson (1961). Levana, Geneseo, Middlesex, Rhinestreet, Pipe This interpretation is based on geochemical attri- Close examination of these black shales reveals Creek, and Dunkirk Shales. The Dunkirk Shale is butes believed to be indicative of deposition in a number of features that indicate reworking equivalent to the basal part of the Ohio and Chat- an anoxic water column, and the assumption that and bedload sediment transport by waves and tanooga black shales, which overlie and onlap such fine-grained, organic-rich sediments could currents consistent with a shallow-marine inter- unconformities farther to the west. Total organic only accumulate in deep, low-energy settings pretation. The presence of fossils and bioturba- carbon (TOC) measurements and regional cor- like the modern Black Sea. tion within the black shales (McCollum, 1988; relations show that each of these black shales The observation that many black shales in Wilson and Schieber, 2015) show that the water displays a consistent pattern: they thin, become the rock record directly overlie unconformities column could not have been permanently anoxic. more organic-rich, and onlap and pinch out on led some early workers to interpret a shallow- The objectives of this paper are (1) to show that unconformities developed on the tectonic highs to marine origin (e.g., Grabau and O’Connell, these black shales onlap unconformities devel- the west (Smith, 2013). These well-documented 1917). Conant and Swanson (1961) observed oped on a tectonic high to the west, and (2) to unconformities are recognized by sharp-based that the organic-rich Devonian Chattanooga present a model of shallow-marine deposition shale packages, missing section, thin lag deposits, Shale of the southern Appalachian Basin directly for the Devonian black shales of the northern and low-angle truncations (Schieber, 1998; Over, overlies a major unconformity and was time- Appalachian Basin. 2007). These sharp discontinuities can also be equivalent to nearby areas of exposed land, recognized in the subsurface using wireline logs which led to the interpretation that the shales OBSERVATIONS (Smith, 2013) where there are sharp contacts and could not have been deposited in any more than The study area is in the northern Appala- missing section. For the sake of brevity, we focus 30 m of water. Nonetheless, in many subsequent chian Basin in central and western New York the discussion on the characteristics of the Gen- studies, deep-water interpretations were favored. State (Fig. 1A), which was bounded to the east eseo Shale where the onlap is very clear (Fig. 2), Water-depth estimates for the deposition of black by the active Acadian orogenic front and to the noting that the other black shales mentioned *E-mail: [email protected] CITATION: Smith, L.B., Schieber, J., and Wilson, R., 2019, Shallow-water onlap model for the deposition of Devonian black shales in New York, USA: Geology, v. 47, p. 1–5, https://doi.org/10.1130/G45569.1. Geological Society of America | GEOLOGY | Volume 47 | Number 2 | www.gsapubs.org 1 progressively younger toward west as they onlap and pinch out (modified from Rogers et al., 1990). Lst—Limestone. 1990). etal., Rogers from out(modified onlapandpinch asthey west toward younger progressively become interval shale black successive of each ages Basal State. York New for stratigraphy Devonian Upper to Middle B: VA—Virginia. Virginia; A Figure 1. A: Map of Appalachian Basin, eastern United States. Study is focused on New York (NY). PA—Pennsylvania; OH—Ohio; WV—West WV—West OH—Ohio; PA—Pennsylvania; (NY). York onNew isfocused Study States. United eastern Basin, Appalachian Mapof A: 1. Figure 2 (line A-A York New central to western from 1)thatextends (Fig. Shaleinterval Middlesex to Limestone Tully sectionofonlapping cross Wireline-log 2. Figure at base of Middlesex Shale. Lst—Limestone. Shale. at baseofMiddlesex andone it, oneabove Limestone, Tully onebelow inred: unconformities sectionshows Cross density(BD[g/cc]). (GR [APIunits])andbulk W A 02 A onlap GR 1 138 40 est 03 0 2 3 0 0 2 2 0 0 2 1 0 0 BD 7 Mainly GrayShale Rhinestreet ′ ; see inset). Datum for cross section is base of Middlesex black shale and interpreted equivalents to east. Logs are gamma ray gammaray Logsare east. to equivalents shaleandinterpreted black sectionisbaseof Middlesex cross for Datum seeinset). ; 02 GR onlap 2339 40 A’ 03 0 2 9 0 0 2 8 0 0 2 7 0 0 2 6 0 0 BD 5 M iddlese x Limestone 02 onlap GR 2223 40 B 03 0 0 3 3 0 0 3 2 0 0 3 1 0 0 3 0 0 0 2 9 0 0 BD 100m 1 Fir 02 GR Tr 2212 40 03 ee Ls 0 0 4 0 0 0 3 9 0 0 3 8 0 0 3 7 0 0 3 6 0 0 BD 6 Organic-Rich Shale Genundewa Lst www.gsapubs.org t 02 GR 21875 40 Geneseo Shal 03 4 5 0 0 4 4 0 0 4 3 0 0 4 2 0 0 4 1 0 0 4 0 0 0 3 9 0 0 3 8 0 BD Lo 02 GR di Ls | 22978 40 Volume 47 Volume 03 400 0 4 7 0 0 4 6 0 0 4 5 0 0 4 4 0 0 4 3 0 0 4 2 0 0 4 1 0 0 4 0 0 0 BD t e T ully Limestone | Number 2 Unconf Organic-rich shalesgradelaterally 02 GR 22861 40 03 2 0 0 3 1 0 0 3 0 0 0 2 9 0 0 2 8 0 0 2 7 0 0 2 6 0 0 into grayshalebasin 2 5 0 0 2 4 0 0 2 3 0 0 2 2 0 0 2 BD | ormit GEOLOGY Fir 02 GR y Tr 23134 40 03 ee Ls 2 2 0 0 2 1 0 0 2 0 0 0 1 9 0 0 1 8 0 0 1 7 0 0 1 6 0 0 1 5 0 0 1 4 0 0 1 3 0 0 1 2 0 0 1 1 0 0 1 0 0 0 BD | Geological SocietyofAmerica t ward 02 GR 23146 40 03 2 6 0 0 2 5 0 0 2 4 0 0 2 3 0 0 2 2 0 0 2 1 0 0 2 0 0 0 1 9 0 0 1 8 0 0 1 7 0 0 1 6 0 0 1 5 0 0 1 4 0 0 1 3 0 0 1 2 BD 02 GR 2297 Eas 400 A’ 2 4 0 0 2 3 0 0 2 2 0 0 2 1 0 0 2 0 0 0 1 9 0 0 1 8 0 0 1 7 0 0 1 6 0 0 1 5 0 0 1 4 0 0 1 3 0 0 1 2 0 0 1 1 0 0 1 0 0 0 9 0 0 BD 4 t 3 above show a comparable pattern (see Fig.