Date of Award

2002

Document Type

Thesis

Degree Name

Master of Science (MS)

Department

Geology

First Advisor

W.D. Gosnold Jr.

Abstract

There are roughly 160 known impact craters listed in the Cratering Database (Earth Impact Database, 2002) of these approximately 20% are buried structures. Geophysical methods of investigation are used to study buried impact structures and add to the growing knowledge base of crater properties. Approximately three to five craters are discovered each year, some of which may be identified through gravity anomalies. A high-resolution gravity survey of the Lake St. Martin impact structure, southern Manitoba, Canada, was conducted to further interpret the structure's gravity anomaly. Previous studies have suggested two different crater diameters for Lake St. Martin, one of 23-km and one of 40-km. The residual Bouguer gravity from this study displays a concentric negative anomaly 40 km in diameter, a distinct change in amplitude approximately 23 km in diameter, and a positive anomaly over the central peak.

Data from six hundred and ninety-nine gravity stations were recorded using a LaCoste and Romberg gravity meter. The majority of the stations were placed along existing roads at intervals of 0. 1, 0.2 or 0.4 miles, depending on their distance from the base stations, in order to cover as much area as possible within a three hour interval required for meter corrections. Additional data were collected using specialized transportation.

Residual Bouguer gravity maps were created by subtracting a regional trend surface from corrected observed gravity values. The gravity survey defines the crater as having a circular negative Bouguer gravity anomaly of irregular peak intensity, -13 .5 mGals amplitude along the E-W profile and -16 mGal amplitude along a SE - NW profile extending slightly beyond the uplifted annular ring of granite. The central uplift is reflected in the gravity signature as an uneven positive 5 mGal anomaly approximately 6.8 km wide. A 2D forward model of two gravity profiles indicate density contrasts between rock types, brecciation, and fracturing of sediments and bedrock. The gravity signature shows irregularities in its profile due to possible fault blocks along a terrace zone extending out to approximately 40 km.

Download

Included in

Geology Commons

Share

COinS