The North structure: evidence for a second possible impact event at the Bloody Creek site, Nova Scotia, Canada

Ian Spooner, Peir Pufahl, Trevor Brisco, Jared Morrow, Mariella Nalepa, Peter Williams, George Stevens

Abstract

The North structure is a discontinuous, partially flooded elliptical basin 250 m in diameter and defined by arcuate scarps. It is located in Annapolis County, Nova Scotia, approximately 1 km north of the Bloody Creek structure, a possible 400 m-diameter elliptical impact crater. Geophysical surveys indicate that raised scarps border a broadly elliptical basin with depth/diameter ratios similar to those at the Bloody Creek structure. Percussion coring and probing indicated that the basin is in-filled with 3.5 m of lacustrine sediment and peat overlying post-glacial alluvial sediment and diamicton. Samples collected proximal to the rim of the structure contain kink-bands in feldspar and biotite and possible planar microstructures in quartz and feldspar. The elliptical nature and similar, anomalous morphometries of the North and Bloody Creek structures indicate that two, low-angled, genetically linked impacts may have taken place at the site. Both structures are interpreted to be post-Pliocene (<2.6 Ma), based on the unlikelihood of their preservation during Cretaceous-Paleogene regional peneplanation.

 

“An approximately 0.4 km diameter elliptical structure formed in Devonian granite in southwestern Nova Scotia, herein named the Bloody Creek structure (BCS), is identified as a possible impact crater. Evidence for an impact origin is based on integrated geomorphic, geophysical, and petrographic data. A near-continuous geomorphic rim and a 10 m deep crater that is infilled with lacustrine sediments and peat define the BCS. Ground penetrating radar shows that the crater has a depressed inner floor that is sharply ringed by a 1 m high buried scarp. Heterogeneous material under the floor, interpreted as deposits from collapse of the transient cavity walls, is overlain by stratified and faulted lacustrine and wetland sediments.

Alteration features found only in rim rocks include common grain comminution, polymict lithic microbreccias, kink-banded feldspar and biotite, single and multiple sets of closely spaced planar microstructures (PMs) in quartz and feldspar, and quartz mosaicism, rare reduced mineral birefringence, and chlorite showing plastic deformation and flow microtextures. Based on their form and crystallographic orientations, the quartz PMs consist of planar deformation features that document shock-metamorphic pressures ≤25 GPa.

The age of the BCS is not determined. The low depth to diameter ratio of the crater, coupled with anomalously high shock-metamorphic pressures recorded at its exposed rim, may be a result of significant post-impact erosion. Alternatively, impact onto glacier ice during the waning stages of Wisconsinian deglaciation (about 12 ka BP) may have resulted in dissipation of much impact energy into the ice, resulting in the present morphology of the BCS” (ABSTRACT – Spooner et al 2009).

Bloody Creek structure is an elliptical 0.35 x 0.42 km diameter NW trending depression, 10 m deep, rimmed by a continuous low scarp and filled with lacustrine and wetland sediments. The structure formed within the medium- to coarse-grained biotite monzogranite of the Scrag Lake pluton, the older group of the 388-372 Ma South Mountain Batholith. The area has been glaciated, has a low relief and is mostly covered by <5 m thick till, wetland, lake and forest. The structure is now flooded by a dam. The gallery below is a series of images taken as we approached the Bloody Creek structure from the north. The approximate location of the impact is indicated in each image.

From the Journal of the Royal Astronomical Society of Canada – 1995

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“Petrographic, geophysical and geomorphic evidence indicate that the BCS is likely an impact crater. The elliptical crater rim is not a common feature and may be an indication of oblique impact (or a double-bolide impact – C, O’Dale). The relatively shallow depth of the BCS is anomalous and, together with the high shock-metamorphic pressures recorded at the rim of the crater, may indicate that significant post-impact erosion has taken place. Alternatively, impact onto thin glacier ice overlying bedrock may have resulted in the low relative depth of the BCS” (CONCLUSIONS – Spooner et al 2009).

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