New York State Geological Association Fieldtrip Guidebook 80th Annual Meeting Lake George, New York September 26-28, 2008 above: Sketch of the great trap dyke at Avalanche Lake. From Emmons, 1838 below: The ‘Great Unconformity’. ca. 510 Ma Cambrian Potsdam Sandstone overlying ca. 1100 Ma Proterozoic gneiss, Putnam Center, NY Sponsored in part by: New York State Energy Research and Development AuthoritySTRATIGRAPHY, SEDIMENTOLOGY AND DIAGENESIS OF THE POTSDAM FORMATION, SOUTHERN LAKE CHAMPLAIN VALLEY, NEW YORK Bruce Selleck Department of Geology Colgate University Hamilton, NY 13346 INTRODUCTION The Cambrian Potsdam Formation is widely exposed on the periphery of the Adirondack massif in northern New York State and adjacent Ontario and Quebec. As the basal unit of the Paleozoic sequence over the cratonic interior of eastern Laurentia, its distribution and character are strongly controlled by the geometery of the pre-Potsdam erosional surface, and patterns of rift-related faulting of the Iapetean margin and sea-level change. In the northern Lake Champlain Valley, the Potsdam is characterized by a basal arkosic unit, the Ausable Member, which is overlain by marine quartz sandstones of the Keeseville Member (Fisher, 1977) Recent study of key exposures in the Lake Champlain Valley region (Landing, 2007) have more firmly established age relationships of the Keeseville and Ausable Members of the Potsdam. Landing (2007) has also documented the existence of a Middle Cambrian unit, the “Altona” Formation, which underlies the Ausable Member of the Potsdam Formation in the northern Lake Champlain region. The Potsdam Formation is approximately time-equivalent to the Danby and Winooski Formation of western Vermont (Mehrtens and Butler, 1987; Landing, 2007). The Potsdam is thickest in the northern Champlain Valley and adjacent southern Quebec (~250 meters), likely related to accumulation on the downthrown side of an Iapetan margin fault (Landing, 2007). In the area of this field trip, the total thickness reaches a maximum of 80 meters. A locally-developed basal arkosic unit may be correlative to the Ausable Member. The succeeding quartz-rich sandstones are referable to the Keeseville Member. The Potsdam Formation acted as an aquifer for the transport of basinal brines and hydrothermal diagenetic fluids during burial. Significant diagenetic alteration of the original sediment has occurred, and the Proterozoic basement rocks beneath the Potsdam have been altered by hydrothermal fluids (Whitney and Davin, 1987). This alteration may be related to seismic pumping of fluids during Taconic (Late Ordovician), Acadian (Middle-Late Devonian) and/or Alleghanian (Carboniferous-Permian) tectonism on the Laurentian margin. In the southern Lake Champlain Valley and Lake George Valley, the Potsdam records non-marine, tidal flat and nearshore marine depositional systems. This field trip will include exposures of a variety of sedimentary facies within the Potsdam Fomration, and will also examine relationships with underlying Proterozoic basement and burial diagenetic and hydrothermal features within the sandstone and basement rock. STRATIGRAPHY AND SEDIMENTOLOGY OF THE POTSDAM FORMATION IN THE SOUTHERN LAKE CHAMPLAIN VALLEY Lower Potsdam Formation (=Ausable Member?) The Potsdam Formation, in the area of this field trip, includes a basal sequence of arkosic sandstone and conglomerate that reaches a maximum thickness of ~30 meters. The succeeding marine quartz sandstones of the Keeseville Member reach a maximum of ~50 meters. The basal arkosic unit is discontinuous in distribution, with the most extensive and thickest development limited to the area south of Ticonderoga Village along NYS Route 22 (Figure 1). Outside of this area, the Keeseville Member rests directly on Proterozoic basement. The basal arkosic unit is dark green to gray-green or yellowish green in color, massive to thick-bedded, irregularly laminated, cross-laminated fine pebble conglomerate and sandstone. Reverse and normal graded beds 5-20 cm thick are common, and trough cross-beds are present. This unitFigure 1 - Location Map (from Google Maps) resembles the Ausable Member of the Potsdam Formation in the northern Lake Champlain Valley. The depositional environments of the basal arkosic unit have been interpreted as alluvial fan and braided stream, based upon sedimentary textures, bedded thickness and primary structures (McCrae, et al, 1986). The source area of the sediment was local, since clasts of granitic and calc-silicate gneiss are common in the conglomerates of the basal unit. The age of the basal arkosic unit in the southern Lake Champlain Valley is not well-constrained. Fisher (1977) proposed that the Ausable Member and related basal arkosic faciesof the Potsdam Formation might be late Proterozoic (Hadrynian) in age, based on the assumption that these facies accumulated in rift basins developed on the margins of the opening Iapetus Ocean. McRae, et al (1986) used paleomagnetic techniques to determine a poorly-cosntrained early Paleozoic depositional age for the Ausable member and other basal Potsdam Sandstone units in the northern New York State outcrop belt. However, the intense post-depositional alteration of the Potsdam Sandstone suggests that the primary depositional paleomagnetic signal was not preserved, and that the remanence measured is likely a diagenetic artifact. The recentdescription of a trilobite-bearing unit, the early-middle Middle Cambrian “Altona Formation”, beneath the Ausable Member in the northern Lake Champlain Valley (Landing, 2007), requires a Middle Cambrian age for type Ausable Member. In the southern Lake Champlain Valley, quartz sandstones and carbonates of the marine Keeseville Member of the Potsdam contain a uppermost Middle Cambrian trilobite fauna (Landing, 2007). The marine facies of the Potsdam in this area contain local beds of coarse, locally derived clastics, and the contact between the basal arkosic facies and overlying Keeseville is apparently conformable, and represents a marine transgressive surface. These factors suggest relatively continuous deposition during the time interval represented by the basal arkosic unit and overlying marine Keeseville Member, and thus a middle-late Middle Cambrian time of deposition for the basal unit is suggested (Figure 2). The green basal Potsdam facies contains abundant iron-rich