Revised stratigraphy of the Trenton Group in its type area, central New York State: sedimentology and tectonics of a Middle Ordovician shelf-to-basin succession

Abstract This paper presents new stratigraphic correlations of the middle and upper parts of the Trenton Group in the type area, near Trenton Falls, New York, based on detailed bed by bed matching, of all outcrop sections. This work, in conjunction with newly revised biostratigraphy and geochemical fingerprinting of K-bentonites, has been used to establish a high resolution chronology for these deposits. Our revised correlations reveal that published stratigraphic–geochronologic schemes are largely in error, resolve several long-standing dilemmas, and have important implications for interpreting sedimentological and tectonic history of the Taconic foreland basin. Key new conclusions/revisions include: (1) The lowermost part of the Trenton type section at Trenton Falls is laterally equivalent to the Rathbun Member of the Sugar River Limestone (lower Shermanian) in the Newport–Herkimer, New York area. (2) The medial Trenton (Denley Formation), dated primarily within the Corynoides americanus graptolite Zone, can be divided in ascending order into two distinctive units, the Poland, Russia members, each of which is further subdivisible into component shallowing-upward cycles and condensed beds. As such, the Poland is completely exposed at Trenton Gorge (contrary to assertions by previous authors) and is about 10.5 m-thick. At its type section, also Trenton Gorge, the overlying Russia Member, comprising four shallowing-upward cycles, extends upward from the Kuyahoora K-bentonites for about 24 m to its sharp upper contact with another distinctive and fingerprinted K-bentonite, the High Falls ash bed. (3) Both the Poland and Russia members thin southeastward from Trenton Falls and become condensed in downslope sections near Middleville. However, the Poland section then thickens and passes eastward into basinal dark gray shales (lower-medial part of the Flat Creek Formation) in central Mohawk Valley sections, whereas the Russia remains thin and relatively carbonate-rich throughout this area. (4) A third unit, the Rust Limestone is elevated to formation status and subdivided into members. The lower part of the Rust Formation (Mill Dam Member) thins dramatically to the southeast from about 12 m at Trenton Falls to 1.5–2 m in the Middleville–Herkimer area before thickening again into basinal black shale facies. (5) The upper Rust and Steuben formations (coarse skeletal pack-to grainstone facies) of the Trenton Falls area apparently thin by condensation into the Newport area before expanding again into turbiditic slope facies of the Dolgeville Formation (essentially corresponding to the Orthograptus ruedemanni graptolite zone) beginning in the Middleville–Herkimer area. The new correlations imply that the lower-middle Rust interval belongs to the Corynoides americanus graptolite Zone, and that the upper Rust–Steuben interval probably belongs in the O. ruedemanni Zone, rather than the Climacograptus spiniferus or even to the lower Geniculograptus pygmaeus Zone, as previously inferred. (6) The Dolgeville carbonate turbidite facies is found to extend eastward to the vicinity of the Hoffmans Fault, east of Amsterdam. (7) Slumped breccia-filled channels in shelf-margin facies of the upper Rust and Steuben limestones may have served as feeder conduits to submarine fans now represented by the Dolgeville Formation. These observations indicate that a sediment-starved east-facing submarine ramp was developed across the study area during Shermanian time. Regional lithospheric flexure coupled with westward retreat of the shelf, explains the distribution of condensed facies and discontinuities. The widespread distribution of many marker beds plus the observation of spectral facies gradations at many levels, suggests that submarine faulting was usually a minor process superimposed on larger-scale diastrophic and eustatic patterns.