Shallow-water onlap model for the deposition of Devonian black shales in New York, USA: REPLY

Abstract

We welcome the discussion of our paper (Smith et al., 2019) by Ver Straeten et al. (2019), because all of the authors contributed significantly to our understanding of these Devonian strata. One novel aspect of our approach, however, is the integration of hundreds of wireline logs along with drill core and outcrops, which allows us to view full sections of these strata in three dimensions across the basin rather than trying to piece things together from limited outcrops.The authors make several points in their discussion, and we will show how these align with the shallow onlap model. For starters, we are encouraged that Ver Straeten et al. now advocate a >50 m or 50-150 m water depth for these shales, because for many years the dominant paradigm posited water depths in excess of 150 m. We consider this progress. At 50 m, we barely have an argument.Ver Straeten et al. write that the black shales are "concentrically bounded" by gray shales with shallower marine fauna. If they mean by this that gray shale occurs as lateral equivalents on all sides of the black shales, this is simply not true. Gray shale does occur to the east of the black shales, but one of the main reasons why we think these black shales are shallow marine facies is that our detailed three-dimensional correlations show that they directly onlap unconformities to the west without transitional facies.High-amplitude sea-level fluctuations have been called upon to explain the presence of relatively shallow marine limestones within the Marcellus Shale (Ver Straeten et al., 2011). Yet, high-amplitude sea level fluctuations are typical of icehouse conditions, whereas the Middle Devonian and most of the Late Devonian accumulated under greenhouse conditions (National Research Council, 2011, their figure 1.2) when glacioeustatic sea level fluctuations would have been at a minimum. Witzke (2011) determined that third-order sea level changes of 15-35 m occurred during deposition of the interval of interest with higher amplitude changes occurring later in the Devonian. Sea-level changes of that magnitude are consistent with our interpreted water depths of a few tens of meters. We also note that light penetration (or lack thereof) is not merely a function of water depth. Water clarity often fluctuates seasonally and can be affected by storms, turbidity, runoff, and algal bloom events.Wireline log correlations of the stratigraphy above and surrounding Onondaga pinnacle reefs show that some of them do extend to the top of the Marcellus but there is a complete section of Skaneateles overlying all of them. The thickness of some of the reefs in part appears to be accommodated below the Onondaga, likely due to evacuation of underlying Silurian salt. Most of the pinnacle reefs extend upward less than 30 m into the Marcellus, with the highest extending ~35 m. The pinnacle reefs were likely exposed during deposition of at least some of the Marcellus, but even if they weren't their height does not force an interpretation of more than a few tens of meters water depth.Ver St...

Metal-rich organic matter and hot continental passive margin: drivers for Devonian copper-cobalt-germanium mineralization in dolomitized reef-bearing carbonate platform

“…Under greenhouse conditions in the Late Devonian (Fig. 1 ), glacio-eustasy is non-existent and third-order sea level changes of 15–35 m in the Middle to Late Devonian are documented (Witzke 2011 ; Smith et al 2019a , b ; Kabanov and Jiang 2020 ) although sea-level changes up to 90–145 m from the late Middle Devonian (Givetian) to Late Devonian (Famennian) may have been possible (Witzke 2011 ; Ver Straeten et al 2019 ). Therefore, in this context and considering the “fluid pumping” model (Shalev and Yechieli 2007 ; Vasyukova and Williams-Jones 2022 ), we conceptualize that succeeding transgression-regression cycles at the climax of Devonian greenhouse conditions may have contributed to the dynamics of flow of pulsed mineralizing fluids for sediment-hosted Cu-Co sulfide mineralization in reef-bearing carbonate platform.…”