A6: QUATERNARY DEGLACIATION OF THE CHAMPLAIN VALLEY WITH SPECIFIC EXAMPLES FROM THE AUSABLE RIVER VALLEY
David A. Franzi
Center for Earth and Environmental Science
SUNY Plattsburgh
Plattsburgh, NY 12901
David J. Barclay
Geology Department
SUNY Cortland
Cortland, NY 13045
Rebecca Kranitz and Kelly Gilson
INTRODUCTION
Deglacial events and paleoenvironments in northeastern New York were influenced by the high relief of the Adirondack Mountains and predominance of north to northeast-draining river valleys (Figure 1). Landforms and sediments within these valleys record sequences of proglacial lakes that existed between the retreating margin of the Laurentide Ice Sheet (LIS) and drainage divides farther south. In general, these proglacial lakes evolved from small headwater impoundments to lowland lakes as the LIS exposed connecting spillways, and each change in lake level caused concomitant changes in tributary stream channels as river systems adjusted to the drop in local base level. Because of the interdependence of these water bodies and the adjacent LIS margin, the chronology of these lakes forms a key record for constraining the timing and style of deglaciation of this area.
In this trip we will examine the field evidence for these lakes and consider the implications of their chronology. In particular, we will focus on the methods used to reconstruct the sequence of proglacial lakes in the Ausable River valley and the constraints that this lake chronology places on a putative interval of post-LIS alpine glaciation in the Adirondacks. These deglacial events and deposits have influenced the evolution of modern drainage patterns, and we will also consider the geomorphology of Ausable Chasm and the nearby Ausable River delta at the shoreline of Lake Champlain as they relate to late Pleistocene and Holocene landscape evolution.

A6: QUATERNARY DEGLACIATION OF THE CHAMPLAIN VALLEY WITH SPECIFIC EXAMPLES FROM THE AUSABLE RIVER VALLEY

Meeting point

Alternate Meeting Point

STOP 1. Chapel Pond, Keene Valley, NY
Chapel Pond Pass (elev. 505 meters a.s.l.) served as the outlet for proglacial Lake Chapel. We shall use this scenic spot to present a brief summary of the field trip, regional geography and deglacial lake chronology of the Ausable Valley.

STOP 2. Styles Brook Mouth, Jay, NY
We shall stop briefly near the mouth of Styles Brook to discuss Tropical Storm Irene, the damaged it caused, the restoration efforts and natural recovery of the fluvial system. We may also use this opportunity to consolidate vehicles to reduce parking congestion at the next stop.

STOP 3. Styles Brook Valley Bottom, Jay, NY
We will access the Styles Brook study area via a trail that crosses private property. Time constraints prevent us from viewing the entire study reach but the flood damage and late Pleistocene sediment exposures at this location are typical.

STOP 4. Jay Covered Bridge, Jay, NY; Discussions and Lunch Stop
The covered bridge in Jay lies adjacent to a knickpoint formed by a metanorthosite outcrop in the East Branch channel. We shall take advantage of this scenic location to enjoy lunch and discuss our paleogeographic reconstructions and implications of Styles Brook stratigraphy and landforms for alpine glaciation.

STOP 5. Clintonville Pine Barren, Clintonville, NY
The Clintonville pine barren is situated on a large deltaic sandplain built by the Ausable and Little Ausable rivers into an embayment of glacial Lake Coveville in the lower Ausable Valley (Figures 4B and 4C). The Little Ausable River was originally a tributary of the Ausable River but the lowering of glacial Lake Vermont water level to the Fort Ann stage initiated downcutting into the Coveville delta deposits and resulted in its diversion across the Ausable drainage divide to its present course (Figure 4D). The Clintonville pine barren is a fire-dependent ecosystem, consisting primarily of pitch pine with an understory of heath plants that has adapted to the low nutrient and drought-prone soils of the deltaic sandplain.

STOP 6. Clintonville Stream Terrace Deposits, Clintonville, NY
The upper portion of the north face of this small gravel pit exposes about 4m of medium to thickly bedded pebble gravel, sand and minor silt that is part of a stream terrace graded to the lower glacial Lake Fort Ann delta at Keeseville. Individual beds range from about 0.1 to 1.0 meters thick and are traceable for several meters across the exposure. The middle approximately 2–3 meters of the section contains a remarkable set of climbing dunes, the upper and lower bedding surfaces of which dip gently upvalley.
The stream terrace gravel and sand disconformably overlies a discontinuous (0.0–0.7 m thick) layer of reddish brown rhythmically laminated silt and clay (varves). The undisturbed clay section is inaccessible but estimates from photographs indicate that it contains at least 20 silt-clay couplets. The west wall of the pit is oriented perpendicular to the paleocurrent and parallel to the valley slope. The rhythmite section in this exposure is thicker (up to a meter thick) and contains soft sediment deformation and slump structures. The rhythmites are draped over 5–6m of sandy, light brownish gray diamicton. The sandy texture of the diamicton is typical of surface till deposits in the region but is markedly dissimilar to the silty, dark gray diamictons observed in exposures at Styles Brook.

STOP 7. Lake Vermont Lacustrine and Deltaic Deposits, Keeseville, NY.
This exposure lies along the south bank of the Ausable River approximately 0.5 mi upstream (south) of the Keeseville Industrial Park (KIP) section that was described by Diemer and Franzi (1988), Franzi et al. (2002) and Franzi et al. (2007) and contains a similar record of lacustrine and deltaic sedimentation. The basal portion of the section consists of approximately 3 meters of dark gray silt and clay rhythmites that probably record pro-deltaic varved clay sedimentation in the Coveville phase of Lake Vermont. Water depth here at this time exceeded 80 meters. Deltaic sedimentation at this site began following the glacial Lake Iroquois breakout when glacial Lake Vermont was lowered to the Fort Ann level. The ice front would have receded about 30 km north to the Cobblestone Hill ice margin over this time interval at an average retreat rate of approximately 0.44 km/yr (Franzi et al., 2007). The locus of deltaic sedimentation shifted nearly 10 km downvalley (eastward) during the breakout (Figures 4C to 4D) from the Coveville-level Clintonville delta at approximately 205 meters a.s.l. to the lower Fort Ann-level delta in Keeseville at approximately 155 meters a.s.l.
The deltaic section at this site exposes approximately 10 meters of lower to middle delta slope foreset and bottomset beds. Excellent examples of ripple-drift cross lamination are commonly exposed.

STOP 8. Ausable Chasm, Keeseville, NY.
Ausable Chasm is a post-glacial gorge cut deeply into the Cambrian-aged Keeseville Member or, to a lesser extent, the Ausable Member of the Potsdam Formation. Landing et al. (2007) note three items of interest in the Cambrian section; 1. the occurrence of scyphomedusae impressions 2. siliciclastic microbial structures and 3. a low-diversity suite of trace fossils. These features are typically restricted to medium to fine grained quartz arenites and rare mudstones in the exposures immediately upstream from the US 9 bridge over the chasm, which are interpreted to represent shallow to emergent littoral sand flat facies (Landing et al., 2007). The scyphomedusae impressions from these outcrops were recently described by Hagadorn and Belt (2008). The outcrops also contain extensive horizontal trackways of Climactichnites and Protichnites (Landing et al., 2007).
A delta complex built by the Ausable and several smaller streams to the northwest overlie the Potsdam sandstones at this location (Figure 4E) (Denny, 1974; Franzi et al., 2007). Forced regression of the Champlain Sea shoreline accompanied isostatic uplift of the region causing the Ausable River to cut through the unconsolidated deltaic sediments and the underlying Potsdam Formation. Incision through the bedrock was probably accomplished by a combination of gradual incision and knickpoint migration and was influenced by the well-developed joint system in the sandstone, resulting in a rectangular channel pattern through the chasm.

STOP 9. Spring Sapping Channels in Champlain Sea Deltaic Deposits.
Spring sapping channels like the two at this site are commonplace in the deltaic deposits surrounding Wickham Marsh. In 1985 the heads of these channels were located several tens of meters farther downstream and their headcuts stood 2 meters high with slight to moderate spring flow near their bases. Since then, the channel heads have migrated headward and the height of the headcuts decreased. We shall discuss the significance of spring sapping as a channel-forming process in the Champlain Sea deltaic deposits and its possible role in the channel avulsion that led to the abandonment of the Wickham Marsh delta lobes and the creation of the modern Ausable delta in Lake Champlain (Figures 11A and 11B).

STOP 10. Wickham Marsh.
Wickham Marsh is a bayhead wetland on Lake Champlain that is separated from the main waterbody by a barrier beach, although nearly all of the beach is presently buried by a railroad embankment. The marsh’s basin was cut as the Ausable River and smaller tributaries adjusted to falling baselevel during the latest stages of the Champlain Sea (Figures 11A and 11B) and possibly early Lake Champlain (Denny, 1974). Spring sapping might have contributed to the development of the tributary stream networks. The lowest deltaic terrace presently lies about 4 meters below the level of Lake Champlain and represents the last deltaic surface formed before the avulsion of the Ausable River to its present course.

STOP 11. Ausable Delta on Lake Champlain.
This scenic spot provides an opportunity to conclude the trip on the modern Ausable delta and summarize the deglacial history of the region.