Storm Dominated Channel Sequences on a Shallow Marine Shelf: Morrowan of Northwest Arkansas

Journal of the Arkansas Academy of Science Volume 48 Article 18 1994 Storm Dominated Channel Sequences on a Shallow Marine Shelf: Morrowan of Northwest Arkansas Kimberley R. Jones University of Arkansas, Fayetteville Doy L. Zachry University of Arkansas, Fayetteville Follow this and additional works at: http://scholarworks.uark.edu/jaas Part of the Geology Commons, and the Stratigraphy Commons Recommended Citation Jones, Kimberley R. and Zachry, Doy L. (1994) "Storm Dominated Channel Sequences on a Shallow Marine Shelf: Morrowan of Northwest Arkansas," Journal of the Arkansas Academy of Science: Vol. 48 , Article 18. Available at: http://scholarworks.uark.edu/jaas/vol48/iss1/18 This article is available for use under the Creative Commons license: Attribution-NoDerivatives 4.0 International (CC BY-ND 4.0). Users are able to read, download, copy, print, distribute, search, link to the full texts of these articles, or use them for any other lawful purpose, without asking prior permission from the publisher or the author. This Article is brought to you for free and open access by ScholarWorks@UARK. It has been accepted for inclusion in Journal of the Arkansas Academy of Science by an authorized editor of ScholarWorks@UARK. For more information, please contact , . Journal of the Arkansas Academy of Science, Vol. 48 [1994], Art. 18 Storm Dominated Channel Sequences on a Shallow Marine Shelf: Morrowan of Northwestern Arkansas Kimberly R.Jones and Doy L. Zachry Department of Geology University of Arkansas Fayetteville, AR 72701 Abstract The Brentwood Member of the Bloyd Formation (Morrowan, Pennsylvanian) in northwestern Arkansas contains stratigraphic sequences deposted by tropical storms in middle shelf environments. The deposits are confined to shallow channels incised by strong unidirectional currents into an interval of shale deposited during fair weather conditions. Complete storm sequences reflect initial bottom currents of high competency that declined through time and were succeeded by wave generated oscillatory activity. The storm succession consists of an erosion surface followed by a basal pebble conglomerate, massive grainstone and packstone, whole-fossil wackestone, hummocky cross-strata and a swell lag of platy crinoid calyxes. As storm activity ceased, fairweather deposits of middle shelf clay blanketed the storm sequences. Introduction streams. In recent years modern storm event shave caused storm deposits to be recognized as a normal part of the stratigraphic record. It is assumed that ancient storms would systematically effect coastal areas and adjacent continental shelves, and that these storm deposits would interrupt normal fairweather marine deposition. An understanding of storm processes has heightened the interest in ancient storm deposits and led to more frequenct recognition of such deposits in recent years. Morrowan strata within the Bloyd Formation of northwest Arkansas contain depositional characteristics that can not be attributed to normal open marine, inner shelf depositional processes, but are compatible with storm depositional processes. Geologic Setting. Accumulation of early Morrowan sediment in northwest Arkansas occurred in a variety of marine and nonmarine environments on an inner shelf depositional surface inclined to the south at less than .01 degrees. A deeper outer shelf and slope marine environment lay to the south in central and southern Arkansas. Marine strata composed of shale, sandstone, and limestone were deposited in inner and middle shelf settings and dominate the Morrowan sequence. The Hale and Bloyd Formations compose the vlorrowan Series in northwest Arkansas (Fig. 1). The rlale Formation rests unconformably on rocks of vlississippian age. The Bloyd Formation conformably the Hale. Itis divided inascending order into the Jrentwood Member, the middle Bloyd sandstone, the )ye Shale Member and the Kessler Limestone Member Fig. 1). All are marine deposits except for the middle iloyd sandstone, a fluvial interval deposited by braided 1%. ||*^ 3 0 0- f.'^fV.;.) = :;?MIDDLE ;;V BLOYD ;:;:;.;!;:;:¦ j sandstone > -I Q 2 0 0- 2 5 , I I,( BRENTWOOD LIMESTONE gpfegW o °p u- ii^rbJ PRAIRIE GROVE z 10 0- — |>verlies DYE SHALE MEMBER E^??( ij^grffft CANE HILL rfyfl MEMBER ~J~_J 0- rt-TlD <2 Za ° PITKIN LIMESTONE Fig. 1. Stratigraphic column of lower Pennsylvanian (Morrowan) strata of Madison County, Arkansas. Storm channel deposits occur in the upper Brentwood Member. The Brentwood Member is composed of alternating beds of limestone and shale and ranges to 50 feet in thickness (Zachry, 1977). It conformably succeeds the Prairie Grove Member of the Hale Formation. The Prairie Grove accumulated in high-energy, inner shelf environments succeeded by transgression and the deposition of middle shelf shale and limestone beds of the Brentwood Member in slightly deeper water. Deposits Proceedings Arkansas Academy of Science, Vol.48, 1994 84 Published by Arkansas Academy of Science, 1994 83 Journal of the Arkansas Academy of Science, Vol. 48 [1994], Art. 18 believed to be of storm origin have been identified in the upper part of the Brentwood Member (Fig. 1). They were deposited on an erosion surface formed on the middle shelf during maximum storm intensity and accumulated as storm current and wave intensity declined. During the early Carboniferous, the southern part of North America was south of the equator. Arkansas was approximately 20 degrees south of the equator and bounded by an open sea and continential shelf (Smith et. al., 1981). Reconstructions suggest that the shelf was in a belt effected by tropical storm events. Location. The storm channel deposits are in the upper part of the Brentwood Member and are exposed along the east side of Highway 23 approximately 15 miles south of the city of Huntsville in central Madison County (T15N, R26W, Sec. 25; Fig. 2.). Exposures containing storm deposits range from 12 to 15 feet in thickness and extend for a distance of 432 feet. — Discussion Storm Fades. Five storm facies are defined within the storm sequence that is confined above and below by fair-weather shale deposits. In ascending order they are the pebble conglomerate facies, the packstone facies, the wackestone facies, the hummocky corss-stratified facies, and the swell lag facies (Figs. 3 and 4). — 400 FEHT Fig. 3. Cross-section view of storm deposits and channel system. Measured sections include fair weather facies involving the lower shale (LS) and upper shale (US) and storm facies including the basal pebble conglomerate facies (C), athe packstone facies (P), the wackestone facies (W), the hummocky cross-stratification facies (H),and the swell lag facies (L). Fig. 2. Geographic locality map of the study area. Storm channel deposits occur along Highway 23, south of Huntsville in central Madison County. Materials and Methods Outcrop sections were measured with a Jocobs staff and measuring ta (...truncated)