FACIES ANALYSIS OF MIDCONTINENT RIFT SYSTEM IN KANSAS

The Midcontinent Rift System (MRS) formed a half-graben that extends into NE Kansas. The Precambrian rift succession, penetrated by Texaco Noel Poersch#1 (NP#1) well in Washington Co. at depths between 2846 and 11300 ft, comprises two successions. The lower one (11300-7429 ft) is dominated by clastic sediments, and the upper one (7429-2846 ft), composed of mainly volcanic rocks, what suggests a radical change during rift evolution. This study aims to identify lithofacies and facies associations to recognize depositional conditions during rift evolution in Kansas. A detailed facies analysis was carried out in discontinuous cores retrieved from the lower, clastic succession (5395-11300 ft deep). Facies attributes (lithology, grain size, sorting, composition, sedimentary structures, type and degree of cementation) were described at 1:30.5 scale and recorded in a sedimentary log. Sixteen lithofacies were grouped into six facies associations: eolian, fluvial, alluvial fan, fan delta, mudflat, and lacustrine facies associations (F.A.). Changes in the facies associations and bedding orientation, associated with the occurrence of igneous intrusions throughout the studied interval, reflect the dynamic arrangement, with rotation of tectonic blocks during deposition and variable subsidence rates. At the base (11296-10504 ft), fluvial and eolian F.A. are replaced by lacustrine and mudflat ones, forming a transgressive cycle. Between 9965 ft and 8050 ft, a transgressive-regressive cycle is represented by fluvial and eolian F.A., overlain by lacustrine and mudflat, and back to fluvial and eolian F.A. A third transgressive cycle is represented by a change from eolian to alluvial fan and mudflat F.A. between 7568 and 7151 ft, and finally to fan delta and lacustrine F.A. between 5406 and 5395 ft. Recurrent transgressive cycles, frequent igneous intrusions and changes in bedding orientation point to the importance of tectonic efforts as a major control on sedimentation in rift basins. Rift evolution controlled the deposition of the sedimentary package in the MRS in Kansas by increasing subsidence rates and base level during periods of intense tectonic activity that resulted in transgression. Climate changes from arid to humid, and vice-versa, also exerted significant control in F.A.