Abstract
Impact cratering is a fundamental geomorphic process on planetary surfaces. More than 60% of known hypervelocity impact craters on Earth are either partially or completely buried beneath post-impact sediments and one-third have been discovered with geophysical methods. In this thesis, geophysical surveys (gravity, magnetics, seismic, bathymetric mapping) were conducted at the deeply buried (>400 m) Holleford impact crater (~2.35 km) and two probable impact structures (Charity Shoal, Skeleton Lake) in southern Ontario, Canada. 3-D potential field models were constructed to determine the subsurface geology and buried crater morphology, and to evaluate evidence for possible impact versus endogenic origins.
Holleford Crater is a deeply buried, Late Proterozoic-Early Cambrian (ca. 550 ±100 Ma) simple impact crater (~2.4 km) in southeastern Ontario, Canada. Land-based magnetic and gravity surveys and modelling were conducted in this study, recorded a ~ -3 mGal Bouguer anomaly and small (~30 nT) magnetic anomaly over the crater basin. 3-D gravity modelling revealed a deeply buried simple impact basin in Mesoproterozoic basement with an estimated rim-to-rim diameter (D) of 1.8-2 km, a residual rim height of ~20-30 m and true depth (dt) >400 m. The southeast crater rim is dissected by a 150 m deep, 400 m wide erosional channel produced by fluvial rim dissection. The outflow is infilled by >50 m of Late Cambrian clastic sediments, indicating a probable Late Proterozoic to Early Paleozoic impact event.
Charity Shoal is a 1.2-km-diameter, 20 m deep, circular bedrock shoal in eastern Lake Ontario. Marine seismic profiling and total field magnetic surveys (140-line km) were conducted over a 9-km2 area and combined with available multi-beam bathymetric data to evaluate the subsurface geology and structure origin. Seismic surveys revealed ~30 m of Quaternary sediments overlying Middle Ordovician (Trenton Group) carbonates in the central basin and evidence for folding and faulting of the structure rim. Magnetic surveys recorded an annular magnetic high (> 600 nT) and a central magnetic low (~500-600 nT) coincident with a ~-1.7 mGal Bouguer gravity anomaly. The continuity of Middle Ordovician bedrock below the structure rules out a post-Paleozoic intrusion and a pre-Paleozoic intrusion is ruled out with the gravity anomaly. A deeply-buried (> 450 m) impact crater is the only scenario consistent with geophysical evidence. The crater has a rim-to-rim diameter of ~1.2 km, and rim height of ~15-20 m. A 100-m wide breach in the southwestern rim records a possible outflow channel.
Skeleton Lake is a suspected (~4.0 km) Paleozoic-age impact structure in Muskoka, Ontario. The lakebed morphology, subsurface structure and possible impact origin were investigated with high-resolution geophysical surveys (magnetics, bathymetry; ~140 line-km) and 3-D magnetic modelling. Bathymetric data reveal a deep (>65 m) central basin with arcuate (Paleozoic?) bedrock ridges that rise >30 m above the southwestern lakebed. Magnetic surveys recorded a >700 nT magnetic low, which truncates northwest-southeast regional magnetic trends. Low-amplitude, northwest-trending magnetic lineaments delineate basement shear zones below the basin centre. Through-going magnetic lineaments and lack of thermal alteration (e.g., dikes, fenitization) in Mesoproterozoic rocks indicate a volcanic origin is unlikely. A 1.2 km diameter volcanic plug with an Early Cambrian remanence (D = 82.2°, I = 82.7°) can reproduce some aspects of the magnetic anomaly but is at odds with the Bouguer gravity anomaly (~ -3 mGal). Forward modelling of a crater-form basin with induction and remanence magnetization yielded an estimated structure depth of ~1200 m. The basement surface model shows a complex basement topography with no apparent rim structure and elevated ‘pinnacles’ that may represent eroded remnants of a central uplift or a highly-dissected basement topography. The structure apparent diameter (> 4.2 km) and complex basement topography suggest a heavily-modified transitional crater, similar with the Gow (Saskatchewan, Canada) and Kärdla (Estonia) impact structures.
This thesis demonstrates the subsurface exploration of confirmed and suspected impact structures, integrating seismic, potential field (magnetics, gravity) and digital elevation data within a 3-D geophysical modelling workflow. The approach provides important new insights into the surface and subsurface geology, morphology, and post-emplacement modification of the Holleford impact crater, and new geophysical constraints for evaluating two suspected impact structures. Geophysical data confirm that Charity Shoal and Skeleton Lake are deep-seated, crater-form depressions in Mesoproterozoic basement rocks. The weight of geophysical and geological evidence points to impact cratering processes as opposed to an endogenic (volcanic) origin for both structures. / Thesis / Doctor of Science (PhD)
| Identifer | oai:union.ndltd.org:mcmaster.ca/oai:macsphere.mcmaster.ca:11375/27807 |
| Date | January 2022 |
| Creators | Armour, Mary-Helen |
| Contributors | Boyce, Joseph I., Earth Sciences |
| Source Sets | McMaster University |
| Language | English |
| Detected Language | English |
| Type | Thesis |
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