3D OUTCROP MAPPING BY PHOTOGRAMMETRIC METHODS–ASSESSING DEPOSITIONAL AND EROSIONAL EVENTS IN GLACIAL LAKE MISSOULA

Three-dimensional outcrop mapping of marine and terrestrial sequences has proven especially valuable in studies of facies geometry and sequence stratigraphy. Most recently published studies have used ground-based light-detection and ranging (LiDAR) techniques to develop highly accurate 3D clouds of survey points. A separate rectification step of digital photographs can produce 3D models on which mapping is done. While accurate, the instrument costs are substantial; purchase prices can be $50k-200k. Alternative methods to produce precise 3D models of outcrops include digital photogrammetric methods that employ stereo photographs and sophisticated software programs. At least two companies (Adam Technology and 3G Software & Measurement) have developed software packages for use in geotechnical fields for mapping bedding, fracture planes, surfaces, and volumes important to the mining industry and others. We used the AdamTech package in the current research.

A well-exposed, 0.5 km-long, ~15 m-thick section of silty sediments deposited in a lake-bottom position was photographed using a Canon 50D digital camera. The apparent resolution is on the cm-scale over the 0.5 km map distance. A “strip” of overlapping photographs taken from a distance of about 100 m recorded subaqueous sedimentation of apparent varves, debris flows, and scour surfaces above clast-supported, fluvial, boulder-gravel and sands. The lacustrine section fines-upwards from gravel to rippled sand to rhythmically bedded very fine-grained rippled sand, silt, and minor clay. A paleosol developed in eolian loess and lacustrine silts caps the section. Lateral tracing of beds and mapping in the 3D model shows at least two unconformities, highlighted by soft-sediment loading of eroded silts by gravelly sand. Detailed mapping in the 3D model allows determination of dip amounts and truncation directions.

The Garden Gulch section is interpreted to represent transgression of Lake Missoula beds over a stable late Pleistocene landscape. Two or three lake transgressions may be recorded based on the unconformities. The paleosol at the top of the Garden Gulch section can be traced parallel to the current landscape surface, indicating that most of the landscape was formed during the drainage of the last Lake Missoula.