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1/5892 The Leading Edge August 2012

T e r r e s t r i a l i m p a c t

A

3D seismic survey has imaged a complex crater of possibleimpact origin in the Finger Lakes region of upstate New

York near Bear Swamp State Forest. Te Bear Swamp crateris uppermost Ordovician (~444 Ma) in age and is completelyburied in the subsurface at a depth of approximately 1220m (4000 ft). Te nearly circular crater is about 3.5 km (2.2mi) in diameter and contains a central rebound structure

with a diameter of about 1 km (0.6 mi). wo explorationwells were drilled into the crater and core and image-log datawere obtained. Te first well tested the central rebound whichconsisted of steeply dipping beds and heavily brecciated zonesas seen on the image logs. Te second well tested the flank ofthe central uplift in the roughly 300 m (1000 ft) thick annularbasin. Tin sections taken from whole core recovered in the

second well revealed planar deformation features (PDF) inquartz grains within the reworked crater fill sediments. Minorgas shows were encountered at the base of the crater fill sectionin the second well, and both wells encountered significant gas

Evidence for an impact origin of the Late Ordovician Bear Swamp

structure in upstate New York, USADANLEIPHART, Chesapeake Energy Corporation

shows in the fractured target rock beneath the crater. Bothmacroscopic and microscopic evidence from the seismic and

well data suggest this could have been the site of a meteorimpact in a shallow marine to transition zone environment atthe end of the Ordovician Period.

Introduction

Impact structures, or astroblemes, are one the of rarest struc-tures in the geologic record. Presently there are 182 confirmedimpact structures on the planet with roughly two-thirds ofthem evident at the surface (University of New BrunswickPlanetary and Space Science Centre, 2011). Tere has of-ten been speculation that many more exist in the subsurface(Buthman, 1997). Te elusiveness of these buried astroblemes

can largely be attributed to their lack of surface expression cou-pled with scarce subsurface data such as 2D and 3D seismicdata and drilled oil and gas wells. Because of the proprietarynature of most of these data, the possibility exists that there are

Figure 1. Regional map showing the area of the approximately 180 km2 (70 mi2) 3D seismic survey and the proposed Bear Swamp Astrobleme.Te map in the lower left is a time slice through a coherency volume at 650 ms. Note the concentric rings inside the crater. Te red line is theapproximate location of the seismic transect shown in Figure 2 and the schematic geologic model in Figure 7.

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many known or suspected subsurface astroblemesthat have not been publicly identified.

One such potential impact structure has beendiscovered in a 3D seismic survey in the FingerLakes Region of upstate New York (Figure 1). Teprimary exploration targets in this area are the fluvialand deltaic sandstones of the Ordovician Queenston

Formation and the structurally controlled hydro-thermal dolomites of the Black River Formation.During the interpretation of the data, an anomalouscircular structure was observed in the seismic coher-ence volume at the level of the Queenston Forma-tion (Figure 1).

Tis structure, at N4243.187 and W7616.637,is approximately 3.5 km (2.2 mi) in diameter atits top. It is completely buried in the subsurfaceat a depth of about 1220 m (4000 ft). Te seis-mic data show a central uplift area within the cra-ter that rises about 162 m (530 ft) above the base.

Around the central uplift is an annular basin thatis more than 300 m (1000 ft) thick and is charac-terized by synformal seismic reflectors (Figure 2).In three dimensions, this structure has the appear-ance of a complex crater (Figure 3). Te diameterand the depth-to-diameter ratio are consistent withother terrestrial complex craters of impact origin(French, 1998).

Te economic significance of impact structuresis well documented (Donofrio, 1997; Reimold etal., 2005). Hydrocarbons are found in or producedfrom 17 of the confirmed impact structures. Subse-quent to the seismic interpretation, two exploration

wells were drilled to test the Bear Swamp structurefor hydrocarbons and to assist in confirming the im-pact origin theory. Te Atwood 1 tested the centraluplift and the Tilburg 1 tested the flank of the cen-tral uplift in the annular basin (Figure 4). Minor gasshows were encountered while drilling the Tilburgnear the interpreted base of the crater at about 1540m (5050 ft). Significant gas shows were encounteredin both wells at a depth of approximately 152 m(500 ft) below the base of the crater in the Ordovi-cian Lorraine and Utica formations. Te wells hadnot been completed as of this writing.

Geologic context and observations

Te Northern Appalachian Basin was a passive margin fromthe latest Proterozoic until the Upper Ordovician. During thistime, most of the North American craton was a broad, shal-low epicontinental sea. Expansive carbonates were depositedover most of the craton until the upper Ordovician. At thattime the aconic Orogeny uplifted a mountain chain, roughlytrending north-south through present-day eastern New York,and transformed the passive margin into a foreland basin. Teaconic Mountains became the sediment source for the pro-lific Queenston deltaic sequence toward the end of the Ordo-

vician Period (~444 Ma).

Te red shales and sandstones of the Queenston were un-conformably overlain by the fine-grained marine sediments ofthe Silurian Medina Formation. Te Medina Formation canbe mapped across the crater in the 3D seismic survey as an un-deformed event suggesting the crater predates the Silurian. Telack of a raised rim suggests it was removed by erosion duringthe unconformity which places the impact near the end of theOrdovician Period (Figure 2).

Well data reveal the central uplift to consist of steeply dip-ping (up to 55) to chaotic beds that are heavily brecciated inzones (Figure 5d). Te crater fill in the annular basin consists

of alternating sequences of shallow-to-moderately dipping beds

Figure 2.Arbitrary seismic transect through the 3D volume. Te QueenstonFormation horizon marks the unconformity at the top of the crater. Te cratermorphology is shown by the green horizon. Te continuity of the Lockport (above)and the renton (below) removes the possibility of this feature being the result ofeither a processing artifact or of volcanic origin respectively. Note the synforms inthe annular basin around the central rebound structure. Te green boxes on theTilburg wellbore represent the approximate locations of the whole cores takenwithin the crater fill and the subcrater sections. See Figure 1 for the location of thisprofile.

Figure 3. Cutout of the 3D volume with the Queenston top and base craterhorizon (green horizon on Figure 2) extrapolated out to show the 3D morphologyof the complex crater. Note the synformal reflectors on both the inline and crosslinedirections in the annular basin.


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(1015) and brecciated and chaotic intervals. Individual clastswithin the brecciated zones can range up to 1 m (~3 ft) indiameter (Figure 5c). Tese chaotic and brecciated zones con-tinue up nearly 275 m (900 ft) above the base of the crater inthe annular basin. Photomicrographs taken from whole corerecovered in this crater fill section of the Tilburg well revealedthe presence of planar deformation features (PDF) in somequartz grains (Figure 6). Image-log data in both wells show anoverall thinning and flattening of the sediment layers toward

Figure 4. Perspective view of the top Queenston seismic reflector asit then follows the base of the crater (green horizon on Figure 2). Welllocations are shown to visualize the areas of the crater that were tested.

Figure 5. Sample images from the FMI image logs taken in both wells. From top to bottom: (a) Te unconformity at the top of the Ordoviciansection marking the boundary between the crater fill and the overlying Silurian marine sediments of the Medina Formation. Note thebioturbation just below the unconformity (Tilburg). (b) Tinly laminated (~2 cm), nearly horizontal layers near the top of the crater fill abovethe impact related sediments (Atwood). (c) Heavily brecciated crater-fill section showing individual clasts of at least 1 m (~3 ft) in diameter(Tilburg). (d) Chaotic and heavily brecciated zones in the central uplift (Atwood). (e) Soft-sediment deformation features just beneath the craterbase (Atwood). (f ) Representative fractures in the subcrater section within the Lorraine/Utica Formations (Atwood). Depths are measured in feet.

the top of the crater fill (Figure 5b). Near the top of the craterfill is a nearly 15 m (50 ft) thick section where visible laminaeare as thin as 2 cm. Above this is a zone of bioturbation whichis capped by a noted disconformity (Figure 5a). Above this dis-conformity lie the nearly flat marine sediments of the MedinaFormation.

Immediately below the central uplift, bed dips gradually

flatten from about 40 to about 1520. Tis interval is about150 m (450 ft) thick, and exhibits occasional soft-sediment de-formation features (Figure 5e). Below this, the next roughly120 m (400 ft) of sediment down to the limestone of the ren-ton Formation dip fairly consistently at 510. Occasional frac-turing is evident in this interval (Figure 5f).

Interpretation

Observations of seismic and well data are consistent with ashallow marine-to-transition zone impact origin for the BearSwamp crater (Figure 7). Te Queenston sediments of thetarget rock in and around this area consist of red shales and

sandstones and are interpreted as fluvial and deltaic to shal-low marine. Additionally, there appears to be evidence of somesoft-sediment deformation features on the image logs just be-low the crater base. Below that, the deformation continues inthe form of fractures. Tis implies that the target sediments atthe time of impact were still not completely lithified.

Similar environments have been the targets of meteor im-pacts in other parts of the world. In the Ordovician Krdlaastrobleme on Hiiumaa Island, Estonia, more than half of the


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Figure 6. Photomicrographs of PDFs in quartz grains in the Tilburg 1 core (see Figure 2) at (a) 4806.7 ft, (b) 4825.52 ft, (c) 4839.8 ft, and(d) 4849.4 ft. Note in (d) the blocky extinction is caused by metamorphosis of subcrystals within the grain. Te PDFs crosscut the subcrystals,indicating an impact event after the metamorphosis of the grain.

Figure 7. Geologic model of the Bear Swamp structure showing crater morphology based on interpreted 3D seismic data, and geologicinterpretation based on well and seismic data as well as analogous confirmed astroblemes. Patterns within the impact-related sediments indicatezones of chaotic and brecciated sediments. Tis model follows the line of section in Figure 2.

nearly 500 m of crater fill is characterized as shallow marineimpact-related deposits (Suuroja et al., 2002). Tese consist ofslumpback, fallback and resurge breccias, and turbidites which

were the result of intense wave action (tsunamis) in the mo-ments after impact. Other relatively shallow marine impactssuch as Montagnais (Nova Scotia, Canada), Mjlnir (BarentsSea, Norway), Neugrund (Gulf of Finland, Estonia), andvren (vren Bay, Sweden) show similar depositional ele-ments in the crater fill section (Dypvik and Jansa, 2003).

Similarities exist in the Bear Swamp crater (Figure 7) wherechaotic and brecciated zones are evident on the image logsnearly up to the top of the crater fill section. Boulder-sized

clasts, some measuring up to 1 m (~3 ft) in diameter, were

deposited up to 275 m (900 ft) above the base of the craterindicating this thick column of sediments was lain down rela-tively quickly in a high energy environment. Te main agentsof deposition in this environment would have been gravityslumps from the unstable walls of the crater and wave surgesas the shallow sea equilibrated and returned to its pre-impactstate. Te sediments deposited in those conditions are here in-terpreted as impact-related. It is within these reworked impact-related sediments that PDFs were observed in some individualquartz grains (Figure 6). Tese features are typically considereddiagnostic evidence of an impact, as no known endogenic geo-logic process is capable of producing them (French, 2005).

Above these impact-related sediments is an interval of thin


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(approximately 2 cm) and nearly horizontal laminae that re-semble varved sediments in Quaternary lakes including the1.07 Ma impact structure at Lake Bosumtwi in Ghana (Sha-nahan et al., 2006). At the top of the crater fill is a 20 cm (8in) thick interval which appears to contain burrows. Directlyabove this bioturbated interval is the surface of the unconfor-mity at the end of the Ordovician which forms the base of the

Silurian marine transgression.Te lack of a competent target rock and evidence of intense

wave surges may be the primary reason for the absence of araised rim around the structure (Dypvik and Jansa, 2003). Anyremnant of a rim likely was eroded during the unconformity at

which time this may have been the site of a subaerially exposedcrater lake as interpreted from the varve-like layers at the topof the crater fill.

Conclusion

Geophysical and geologic data have revealed a proposed bur-ied meteor impact structure near Bear Swamp State Forest in

upstate New York. Seismic data have imaged the nearly cir-cular crater which has similar dimensions and morphologicalcharacteristics with those of other complex craters formed bybolide impacts. Well-log and core data reveal sedimentationpatterns that have likewise been observed in analogous con-firmed impact structures. Additionally, PDFs found in quartzgrains in the interpreted impact-related sediments provide mi-croscopic evidence of an impact event. Based on all the observ-able data, this impact occurred near the end of the Ordovi-cian Period (~444 Ma) in a shallow marine to transition zoneenvironment.

ReferencesButhman, D. B., 1997, Global hydrocarbon potential of impact struc-

tures, in K. S. Johnson, and J. A. Campbell, eds., Ames Structure innorthwest Oklahoma and similar features: Origin and PetroleumProduction (1995 Symposium): Oklahoma Geological Society Cir-cular, 100, 8399.

Donofrio, R. R., 1997, Survey of hydrocarbon-producing impact struc-tures in North America: Exploration results to date and potential fordiscovery in Precambrian basement rock, in K. S. Johnson, and J. A.Campbell, eds., Ames structure in northwest Oklahoma and simi-lar features: Origin and Petroleum Production (1995 Symposium):Oklahoma Geological Society Circular, 100, 1729.

Dypvik, H. and L. F. Jansa, 2003, Sedimentary signatures and processesduring marine bolide impacts: a review: Sedimentary Geology, 161,

309337.French, B. M., 1998, races of catastrophe; a handbook of shock-meta-

morphic effects in terrestrial meteorite impact structures: LPI Con-tribution No. 954, Lunar and Planetary Institute.

French, B. M., 2005, Impacts in the field: Impact Field Studies Group,volume 2, Winter 2005, 310.

Reimold, W. U., C. Koeberl, R. L. Gibson, and B. O. Dressler, 2005,Economic mineral deposits in impact structures: A review, in C.Koeberl, and H. Henkel, eds., Impact tectonics: Impact Studies, vol-ume 6, 479552.

Shanahan, . M., J. . Overpeck, C. W. Wheeler, J. W. Beck, J. S. Pi-gati, M. R. albot, C. A. Scholz, J. Peck, and J. W. King, 2006,Paleoclimatic variations in West Africa from a record of late Pleis-

tocene and Holocene lake level stands of Lake Bosumtwi, Ghana:Paleogeography, Paleoclimatology, Paleoecology, 242, 287302.

Suuroja, K., S. Suuroja, . All, and . Floden, 2002, Krdla (HiiumaaIsland, Estonia)the buried and well-preserved Ordovician marineimpact structure: Deep-Sea Research II, 49, 11211144.

University of New Brunswick Planetary and Space Science Centre,2010, http://www.passc.net/EarthImpactDatabase/index.html, ac-

cessed December 14, 2011.

Acknowledgments: I thank Chesapeake Energy Corporation for therelease of the proprietary seismic, log, and thin-section images.

Corresponding author: [email protected]

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