A POSSIBLE “NEGATIVE FLOWER STRUCTURE” ON THE TUG HILL ESCARPMENT, NORTHERN NEW YORK

Negative flower structures in the middle Ordovician Trenton and Black River groups have recently been recognized as important reservoirs for natural gas in south-central New York State and elsewhere. The structures are recognized and interpreted in the subsurface using geophysical data. Natural gas reservoirs are interpreted to form as a result of hydrothermal leaching of Trenton and Black River limestones causing them to develop significant pore space (Smith, 2006). The negative flower structure commonly results in the downward “sagging” of the overlying shales of the Utica Formation (Smith, 2006).

On the eastern escarpment of the Tug Hill, near the village of Martinsburg a number of structural and mineralogical features suggest the presence of a partly exposed negative flower structure. In the valley of Rainbow Creek (Lowville 7.5 minute quad.) a SW striking high angle fault cuts Precambrian basement rock as well as middle Ordovician Black River Group Limestones (Johnsen, 1957). The downthrown side is on the SE, but a transtensional component cannot be ruled out. Precambrian quartz-feldspar gneiss near where Rainbow Creek crosses State Route 12 show numerous fractures lined with hydrothermal chlorite and possibly epidote. Along strike to the SW and higher up in the stratigraphic section, Trenton Group limestones are host to galena-calcite veins that were once mined just north of the village of Martinsburg (Glenfield 7.5 minute quad.; Hough, 1860, 1883). Even further along a trend to the SW, in the northwesternmost part of Chimney Point Gulf (of Roaring Brook; Page 7.5 minute quad.) the Utica and Lorraine Group shales and siltstones are inclined 10 to 15 deg. SE in contrast to nearly all other area exposures of Utica and Lorraine group rocks that are essentially flat or gently sloping (<1 deg.) to the SW. These inclined rocks were first described more than 120 years ago (Hough, 1883).

All of these features, together, suggest a larger SW-trending fault-related hydrothermal system in middle Ordovician rocks on the Eastern escarpment of the Tug Hill. The inclined Utica and Lorraine shales and siltstones could represent a portion of a “sag” over hydrothermally leached Trenton and Black River limestones. This partly exposed structure may represent an ideal place to test interpretive models of negative flower structures in south-central NY and elsewhere.