“PALEO-DIPSTICKS” AND “PALEO-PHOTOMETERS”: CONSILIENCE OF SEDIMENTOLOGIC AND PALEONTOLOGICAL EVIDENCE FOR A MODERATE DEPTH ORIGIN OF MIDDLE AND UPPER DEVONIAN BLACK SHALES

Relative water depths under which epicratonic/foreland basin black shales accumulated remains controversial. Recently, there has been resurgent interest in a very shallow water origin for these deposits. Certain readily identified dark shales were indeed deposited in shallow lagoonal /tidal flats. However, for the majority of widespread Devonian black shales, diverse lines of evidence from stratigraphy, sedimentology, paleobiology, and geochemistry within and outside of the Appalachian Basin argue that a very shallow-water (<10 m) origin is highly unlikely. Key points include: 1) a complete absence of evidence for subaerial exposure (paleokarst, vadose cements) of limestones underlying black shales, e.g., Devonian Marcellus Sh. over Onondaga Ls. and Geneseo Sh. over Tully Ls. in the classic NY succession; 2) unaltered tops of >30 m tall Onondaga pinnacle reefs in which contacts show a sub-black shale lag; 3) sedimentologic and geochemical data inconsistent with pervasive whole-water-column mixing and sediment surface disturbance in shallow settings due to tides, waves, and storms on short time scales; 4) pronounced manifestations of Milankovitch band- to 10⁶ year-scale sea level variations; 5) Middle Devonian black shales in the Appalachian Basin are basin-centered, concentrically bounded by gradations into gray mudstone and siltstone with gradients of increasingly diverse faunas, indicative of progressive shallowing; 6) onlap relationships of shallow marine facies correlative with black shales over karstified surfaces in the stable craton (Iowa) in which paleo-relief indicates preceding sea level drops of 10s of meters; 7) similarly, onlap of dark silty shallow marine tongues, correlative with offshore black shales, over thick terrestrial redbeds in the Middle-Upper Devonian of the Catskill region, e.g., lower Givetian Moscow and Geneseo fms; and 8) microendolithic borings in brachiopod shells indicative of photosynthetic cyanobacteria in some basin center black shales. Such autotrophs are adapted to low light levels and are distinct from those of shallow water assemblages. These diverse lines of evidence indicate a minimum water depth of 10s of meters for widespread NY Devonian shales, although probably not exceeding 100 m, at least for those showing a presence of photoautotrophic endoliths.