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The AAPG/Datapages Combined Publications Database

AAPG Bulletin


Volume: 47 (1963)

Issue: 2. (February)

First Page: 366

Last Page: 366

Title: Pre-Maquoketa (Middle Ordovician) Cyclic Sediments in the Upper Mississippi Valley Area: ABSTRACT

Author(s): Meredith Eggers Ostrom

Article Type: Meeting abstract


Five cycles are recognized in the pre-Maquoketa (Middle Ordovician) marine strata of the Upper Mississippi Valley area: two in Cambrian rocks and three in Ordovician rocks. A sixth unit, at the base of this cyclic section, shows some cyclic characteristics, but is considered to be of non-marine origin.

A complete cycle includes the following units: Phase 4, carbonate deposits; Phase 3, fine-grained sandstone, fossiliferous, dolomitic, and glauconitic and (or) green shale; Phase 2, interbedded coarse-grained sandstone, locally conglomeratic, fossiliferous, dolomitic, and glauconitic, and poorly sorted sandstone composed of particles ranging in grain size from silt to coarse sand; Phase 1, orthoquartzitic sandstone deposits.

Analogous units in each cycle contain similar attributes of mineralogy, grain size, sorting, fossil content, sedimentary structures, contact relations, geometry, and lateral variations. The thickness of cyclic components may vary and components may be locally absent.

Cycles are believed to represent transgressive depositional environments caused by periodic rejuvenation of the land and inner shelf areas with contemporaneous activity in neighboring basins. Materials delivered to the shelf range in grain size from clay to granules. As land areas were worn down, sediments of Phase 1 accumulated on the shelf, building it upward. As the supply of sediment to the shelf decreased, wave and current energy was diverted toward reworking shelf deposits. Deposits of Phase 2 indicate alternating conditions of low and high energy which produced beds of well sorted coarse- and medium-grained sand alternating with poorly sorted beds of materials ranging in size from silt to granules. During Phase 1 and Phase 2 finer particles were kept in suspension. In Phase 3 t ese finer particles, together with those removed from shelf deposits during reworking, were deposited. Deposits of Phase 3 are fine-grained sandstone and (or) shale which are commonly dolomitic and glauconitic. Intraformational conglomerates are common. Phase 4 is characterized by carbonate deposition believed to have begun when energy conditions reduced sufficiently for the shelf bottom to stabilize and support life.

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