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

• 作者: Charlotte Mehrtens Edwin Romanowicz Henry Maguire Jonathan Kim
• 来源期刊: 地质学期刊(英文)
• 学科: 医学
• 关键词: CAMBRIAN SEQUENCE STRATIGRAPHY PARASEQUENCE Vermont Sedimentary SEQUENCE and Monkton
• 年，卷（期）: 2019,（2）
• 页码范围: 89-108
• 页数: 20页
• 分类号: R73

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 wer