The Cambro–Ordovician System in the Subsurface of Eastern Colorado: A Wireline Well Log Evaluation

Abstract This is a wireline well log study of the subsurface Cambro-Ordovician in eastern Colorado. Seven mappable units, delineated in well logs, are correlated to six formations of western Kansas stratigraphic nomenclature. Five maps and two cross sections have been generated depicting the structure, subcrop, and total thickness of the system. The interval studied is from the base of the Mississippian, here called the Boice Formation, to the top of Cambrian Reagan Sandstone or equivalent. Map A shows major faulting emanating from the Ancestral Apishapa High and the resulting subcrop, which was sculpted by differential erosion on the block-faulted surface. Map B outlines the Eads Axis of the Las Animas Arch which trends into Kansas. A southeast area isopach and cross-section A-A’ illustrate the onlap of the basal Cambro-Ordovician transgression onto the Ancestral Apishapa High. A northeast area isopach defines the erosional edge of the Cambro-Ordovician system against the Transcontinental Arch of the Precambrian craton. Figure 10 and cross-section B-B’ show the author’s correlation and diachronous relationship between the Cambrian Reagan Sandstone and the Cambrian Bonneterre Formation sand in the northeastern part of Colorado. The Eads Structural High acted as a facies barrier partitioning the Bonneterre Formation into carbonates to the southeast and sandstone to the northwest of the high.