Using Gamma Emissions to Identify Cycles in the Lower Cambrian Monkton Formation (NW Vermont): Implications for Identifying Sea Level Variation on the Iapetus Margin

摘要

The Monkton Formation is described as a Lower Cambrian regressive sandstone unit containing shallowing-up cycles, called parasequences, which record 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 parasequence architecture and thickness from the successful recognition of facies in the subsurface, information that will clarify how the sea level changes and impacts accommodation space. New geophysical data allow this research project to explore the stratigraphy of the Monkton at higher levels of resolution than previously achieved. Initial statistical study of gamma ray data from a well through the Monkton suggests that meter-scale parasequences 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 cycles were identified in the gamma log, the spatial variation, complexity and variety of parasequence 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 parasequences and lithologies could be characterized by gamma values, outcrops of the Monkton were surveyed containing both parasequences and lithologies that were useful paleobathymetric indicators. The survey determined gamma ray patterns and relative values of a common Monkton parasequence representing tidal flat progradation and a carbonate lithology indicating an abrupt sea level rise. Gamma data from a second well on the University of Vermont campus, along with accompanying borehole camera video, confirmed that the outcrop survey results could be useful in subsurface interpretations of the Monkton. Intervals representing the parasequences and carbonate lithologies were identified in the Champlain College Well and were combined with the UVM Fleming Well findings to create a composite stratigraphic section of the Monkton. The parasequences and lithologic trends identified in the composite section indicated that, over the entirety of the Monkton, accommodation space decreased. The decrease in accommodation space is interpreted to represent a change in the rate of sea level rise. The Monkton Formation deposition occurred in a transgressive systems tract with varying rates of sea level rise, which implied that the overlying Winooski Formation would represent highstand systems tract deposition.