This thesis utilizes several geophysical methods to study the Siljan Ring impact structure, focusing on the western part of the structure. This thesis, and the three papers upon which it is based, reports on attempts to delineate the Paleozoic rocks at depth within the annular ring graben and characterize their structure. In addition, the nature of the basement, which underlies these sedimentary rocks is investigated. Papers I and III focus on analysis of the down-hole logging and borehole core data. As well as the acquisition, processing and interpretation of 2D high-resolution reflection seismic data from the Mora area. The borehole log responses were compared with the core lithology from the Mora 001 borehole and information from two other cores (Mora VM 2 and Mora MV 3) in order to interpret the logs. The logs reveal significant changes in the lithology between boreholes, indicating a very high level of structural complexity, which is attributed to impact tectonics. In addition, the log data revealed a high sonic velocity contrast between the Silurian and Ordovician successions and a higher apparent temperature gradient than in the northern part of the structure. The interpretation of the high-resolution 2D seismic data suggest that the Mora area has been significantly affected by the impact. Several potential faults were identified in the area and interpreted to be post depositional and related to the impact. In paper II, a 2D seismic profile from the Orsa area (12 km) located in the northwestern part of the Siljan Ring was re-processed. To compliment this seismic line, first break traveltime tomography results, vintage seismic OPAB profiles, new and pre-existing gravity data, aeromagnetic data and the bedrock geological map were used to present a geological model along the Orsa profile. Reprocessing of the seismic data resulted in improved stacked and migrated sections and better imaging of the top of the crystalline basement than the original processing. Integrated interpretation of the seismic profiles suggests that the area has been significantly affected by faulting and that the depth to the basement varies greatly along the different profiles.

The purpose of the author is to present a geological survey of the Backbone Ridge area in Burnet and Llano counties, Texas. During the summer of 1947 while enrolled in a geologic field course in McCullough County, it was noted that a new classification of the Cambrian and Ordovician formations In central Texas had been presented In the literature. It was noted at the same time that the Paleozoic rocks of the Llano uplift are more highly faulted than had been shown on previous geologic maps of the region. The writer concluded from field observations that the Backbone Ridge area was more complexly faulted then had been previously shown, and it was decided to test this conclusion by making a detailed geologic map of the area using the stratigraphic subdivisions recently established by Bridge, Barnes, and Cloud. A detailed study was made of these subdivisions and a large number of the type sections were visited. It was necessary to study the complete geologic history of the region in order that the events which involved the complex structural pattern and the present physiographic forms might be properly understood. The material contained in this report consists of data obtained from the literature and from field observations which were made in the area during the months of June and July of 1948. / text