Masters Theses

Date of Award

6-1985

Degree Type

Thesis

Degree Name

Master of Science

Major

Geology

Major Professor

Kenneth R. Walker

Committee Members

Thomas W. Broadhead, Robert E. McLaughlin

Abstract

Approximately 450 meters of Middle Ordovician rocks are exposed at Lay School, Hogskin Valley, Tennessee. This sequence is composed of seven formations, in ascending order: Blackford Formation, Lincolnshire Formation (Elway Member, Hogskin Member, and Upper Member, respectively), Rockdell Formation, Benboit Formation, Wardell Formation, Bowen Formation, and Witten Formation. The top of the Section is cut by the Copper Creek Fault which brings the Cambrian Rome Formantion onto Middle Ordovician Moccasin Formation (the latter not studied here). Detailed petrographic examination of 302 thin-sections of these rocks was made for environmentaily relevant fossil and sediment features selected from taphonomic, paleontologic, and sedimentologic studies. Presence/absence information was collected on 51 such features or parameters and 255 samples were subjected to Q-mode multivariate cluster analysis to reveal groups of samples with similar environmental aspect. The cluster analysis determined seven sample groups of distinctive character here termed ecofacies. Ecofacies No. 1 is characterized by good sorting, fossil fragmentation and abrasion, and horizontal orientation of elongate particles. A sample group containing laminated, cross-laminated, and well-sorted lithologies (Ecofacies 2) also had common horizontal particles. The third ecofacies (Ecofacies 3) with a well-sorted aspect contains common bimodal samples with a larger size fraction of whole and incrusted fossils, a fragmented and abraded smaller fraction, shelter pores, and horizontal allochems. The largest sample group (Ecofacies 4) has frequent bioturbation (which destroyed primary depositional features), as well as micritic features (adhered and infilled micrite, intraclasts, coated grains). Ecofacies 5 and 6 are also poorly-sorted and contain some bioturbation features but also exhibit primary depositional parameters (e.g., shelter phenomena) in addition to micritic features, oncolites (No. 5), and horizontal particles (No. 6). The last, Ecofacies 7, is characterized by pelmatozoan holdfasts, crustose sheets and bound-stones, poorly-sorted and incrusted fossils, and occasional radiaxial cement.

A R-mode or parameter cluster analysis was performed to determine which features co-occurred most frequently in overall distribution. Poorly-sorted fossils, whole fossils or parts, unabraded fossils, and 1-layer incrustations are associated in one group. Coated grains, adhered and infilled micrite, intraclasts, and blackened particles comprised the second group. A third cluster includes well-sorted fossils, pelloids, and fragmented and abraded fossils. Mud-filled burrows, swirled or concentric particles, and spar-filled burrows are features of a fourth group. Shelter phenomena, such as shelter pores, sheltered mud, and perched sediment are associated in the fifth group with unattached crustose bryozoan sheets, articulated bivalved fossils, and fecal pellets. Cement-filled moldic pores occur with 2- and 3-layer incrustations, matrix channel and vug pores (cement-filled), crystal silt, and pelmatozoan holdfasts (sixth group). A large cluster of rare features includes the smaller subgroups of the more common parameters ooids and hemispheroidal oncolites, and convex-up shells clustered with graded bedding. Borings in fossils are associated with particles with 2- and 3-layers of incrustation, and also with bimodal size distributions and oncolites. Planar lamination or layering occurs with horizontal or oriented fossils.

The distribution of ecofacies and feature groups within the Lay School section was used along with Petrographic Fossil Diversity information to reconstruct environments of deposition. The Blackford Formation contains Ecofacies 1, 2, 3, and represents subtidal to supratidal subenvironments present in a coastal tidal flat setting. Ancient sediments consisted of well-sorted sands (Ecofacies 1), laminated muds and silts (Ecofacies 2), fenestral mudstones (Ecofacies 2), and bimodal sands (Ecofacies 3). The Elway Member contains shallowing-upward cycles which represent repeated tidal flat progradation or coastal accretion through the formation of barrier islands and subsequent leeward sedimentation. The progresson of ecofacies in such cycles is from sands of Ecofacies 1 and 3 near the base, to muddy sands of Ecofacies 4 and 5 at the middle, to laminated lithologies and fenestral dismicrites (Ecofacies 2) at the top. The Hogskin Member contains biostromes (stromatoporoid, Ecofacies 7), pelmatozoan sands (Ecofacies 1 and 3), bryozoan boundstones (Ecofacies 7), and other diverse lithologies which reflect deposition in shallowest subtidal offshore environments adjacent to tidal flats. These rocks are followed by an interval (Upper Member, Lincolnshire Formation) characterized by intense bioturbation of mud-supported lithologies (Ecofacies 4). The lower Rockdell Formation has small algal mud mounds (Ecofacies 7) flanked by variably micritic bryozoan/ pelmatozoan sands (Ecofacies 5 and 6). The mounds formed through the interaction of algai binding, bryozoan/pelmatozoan incrustation, early cementation, and sediment baffling. Deposition in an euphotic, above wave-base outer shelf margin is represented by the upper Rockdeil Formation which contains diverse ecofacies. The Benbolt Formation was deposited in a normally quiet water, below wave base setting characterized by a scarcity of algae due to water depth or other factors. This interval has frequent fossil layers of Ecofacies 6, as well as containing Ecofacies 4 and 5. The lower Wardell Formation formed in a nearshore setting of shallow, standing water (Ecofacies 4) adjacent to ephemeral subtidal oncolite channels and shoals of Ecofacies 2 and 5. Shoreward, the tidal flat complexes represented by the upper Wardell, Bowen, and Witten Formations contain mainly Ecofacies 1, 2, and 3.

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