Petrology of Passive Margin-Epeiric Sea Sediments: the Garden City Formation, North-central Utah

• Main Author: Morgan, Susan K.
• Format: Others
• Published: DigitalCommons@USU 1988
• Subjects: petrology; sea sediments; passive margin; garden city; formation; utah; Geology
• Online Access: https://digitalcommons.usu.edu/etd/5623; https://digitalcommons.usu.edu/cgi/viewcontent.cgi?article=6680&context=etd

The Lower Ordovician Garden City Formation is part of the thick sequence of Lower Paleozoic limestones, dolostones, and minor siliciclastic sedimentary rocks of the western United States. The carbonate rocks were formed predominantly by shallow water deposition in tropical, passive-margin epeiric seas. The Garden City Formation is composed of nine lithotypes which represent the various environments. The formation is a storm-influenced transgressive sequence which may be divided into innershelf shallow subtidal and outer-shelf deep subtidal environments separated by a skeletal accumulation. The skeletal accumulation, formed by storm initiation, was a submerged topographic high, below normal wave base. The inner shelf includes the initial peritidal transgressive and shoreface material, which was extensively reworked by storm action, and a patchy distribution of shallow subtidal deposits. It is characterized by shoreward fossil banks and mud mounds, a restricted fauna, large amounts of terrigenous material and repeated occurrences of storm-created intraclastic layers within a nodular limestone. The outer shelf sediments have a diverse fauna, are extensively burrowed and bioturbated, and have significant amounts of chert. Uncommon intraformational conglomerate layers signify deposition below mean storm-wave base. The Garden City Limestone facies were deposited in broad, energy-related zones parallel to the ancient shoreline. These facies were compared to the model of epeiric sea deposition presented by Shaw (1964) and Irwin (1965). There was a lack of evidence within the Garden City sediments to support the existence of an extensive, shoreward, tideless low-energy zone as predicted by the model. The inner shallow subtidal environments remained near normal marine conditions, with water circulation provided by tidal action. Early diagenetic features of the Garden City Formation include compaction, micritization, cementation and neomorphism. Chert formation preceded pressure solution and probably represents silicification of burrows. Dolomitizing fluids moved along faults, unconformities, and bedding planes to selectively dolomitize the formation. Near-surface weathering resulted in dedolomitization and the oxidation of pyrite to hematite.