BEAR SWAMP IMPACT STRUCTURE

BEAR SWAMP IMPACT STRUCTURE

by: Charles O’Dale

  • Type: Simple
  • Location: N 42° 43.187’ W 76° 16.637’
  • Diameter: 3.5 Km
  • Age Ma: ~444a
  • Shock Metamorphism: planar deformation features (PDF) in quartz grains.

a minimum age at the end of the Ordovician period.

Location map showing the area of the ~180 km2 (70 mi2) 3D seismic survey and the proposed Bear Swamp Astrobleme. The map in the lower left is a time slice through a coherency volume at 650 milliseconds. The inset at the lower right is a cutout of the 3D volume with the Queenston top and base crater horizon extrapolated out. courtesy of AGU/Dan Leiphart.
A potential impact structure has been discovered in a 3D seismic survey in the Finger Lakes Region of upstate New York. The Bear Swamp crater is uppermost Ordovician (~444 Ma) in age and is situated within the fluvial-deltaic to shallow marine Queenston Formation.

Possible Impact Origin for the Late Ordovician Bear Swamp Structure in the Finger Lakes Region of New York

Leiphart, D.

Abstract

Impact structures, or astroblemes, are one of rarest formations in the geologic record. Presently there are 176 confirmed impact structures on the planet with roughly two-thirds of them evident at the surface. A potential impact structure has been discovered in a 3D seismic survey in the Finger Lakes Region of upstate New York. The Bear Swamp crater is uppermost Ordovician (~444 Ma) in age and is situated within the fluvial-deltaic to shallow marine Queenston Formation. This nearly circular structure measures 3.5 km (2.2 mi) in diameter and is completely buried in the subsurface at a depth of approximately 1,220 m (4,000 ft).

Seismic data show a central uplift within the crater that rises about 160 m (525 ft) above the base. Around the central uplift is an annular basin that is more than 300 m (~1,000 ft) thick and is characterized by synformal seismic reflectors.

This three-dimensional morphology resembles other complex craters of confirmed impact origin. Two exploration wells were drilled into the crater and image logs were run. The first well tested the central rebound which consists of steeply dipping beds and heavily brecciated zones. The second well was drilled in the annular basin which contains alternating sequences of chaotic zones and shallow dipping beds. Based on analogous impact structures, this crater fill is here interpreted as resurge breccias and turbidites which were the result of intense wave action in the moments after impact. Above these impact-related deposits lies a zone of very thin (~2cm) laminae which resemble varved sediments in lacustrine environments. A bioturbated zone overlies these thin laminae, which is in turn capped by the End Ordovician unconformity. Observations of both seismic and well data are consistent with a shallow marine to transition zone impact origin for the Bear Swamp crater.

Evidence for an impact origin of the Late Ordovician Bear Swamp structure in upstate New York, USA

  • © 2012 by The Society of Exploration Geophysicists

Abstract

A 3D seismic survey has imaged a complex crater of possible impact origin in the Finger Lakes region of upstate New York near Bear Swamp State Forest. The Bear Swamp crater is uppermost Ordovician (∼444 Ma) in age and is completely buried in the subsurface at a depth of approximately 1220 m (4000 ft). The nearly circular crater is about 3.5 km (2.2 mi) in diameter and contains a central rebound structure with a diameter of about 1 km (0.6 mi).

Two exploration wells were drilled into the crater and core and image-log data were obtained. The first well tested the central rebound which consisted of steeply dipping beds and heavily brecciated zones as seen on the image logs. The second well tested the flank of the central uplift in the roughly 300 m (1000 ft) thick annular basin. Thin sections taken from whole core recovered in the second well revealed planar deformation features (PDF) in quartz grains within the reworked crater fill sediments. Minor gas shows were encountered at the base of the crater fill section in the second well, and both wells encountered significant gas shows in the fractured target rock beneath the crater. Both macroscopic and microscopic evidence from the seismic and well data suggest this could have been the site of a meteor impact in a shallow marine to transition zone environment at the end of the Ordovician Period.

GROUND EXPLORATION

This image is from the north rim of the Bear Swamp structure, looking south from the southern end of Skaneateles Lake.