APPLICATION OF GEOPHYSICAL METHODS TO STRATIGRAPHIC PROBLEMS IN THE LOWER CAMBRIAN MONKTON FORMATION: NW VERMONT

The Monkton Formation is described as a Lower Cambrian regressive sandstone unit containing shallowing-up cycles (SUCs) recording tidal flat progradation. Spatial variation and limited outcrops of continuous stratigraphy have made it difficult to characterize how cycles change in architecture and thickness through the entirety of the Monkton Formation. This study seeks to identify stratigraphic trends in the cycle architecture and thickness from the successful recognition of facies in the subsurface, information that will clarify how the sea level changed and impacted accommodation space.

This research project explores the stratigraphy of the Monkton at higher levels of resolution than previously achieved using new geophysical data. Initial statistical study of gamma ray data from a well through the Monkton suggests that meter-scale SUCs are identifiable in geophysical logs. Using continuous gamma log data, this work identified clastic carbonate ratios from a 1034’ deep geothermal well drilled at Champlain College in Burlington, VT. Although SUCs were identified in the gamma log, the spatial variation, complexity and variety of SUC types within the Monkton made it difficult to correlate the sequences with specific environments or changes in sea level.

To see if specific architectural elements of SUCs could be characterized by gamma values, outcrops of the Monkton have been surveyed and SUCs characterized by their gamma ray signatures. The survey was able to determine gamma ray patterns of single cycles, but the subtle variability in clastic/carbonate ratios and cycle types made identifying a single specific cycle in the well log difficult. However, gamma signatures of SUC sets can be used to create a predictive model of how cycle architecture produced by depositional environment changes through the stratigraphy. Gamma data from a second well on the University of Vermont campus, along with accompanying borehole camera video, provided a test for our model. With the precision of the model established, SUC gamma signatures can be used to better identify how changes in depositional environment evolves through the stratigraphy. Work is continuing to create a composite stratigraphic section from which we will be able to identify changes in accommodation space up section.