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The AAPG/Datapages Combined Publications Database

AAPG Bulletin


Volume: 67 (1983)

Issue: 3. (March)

First Page: 513

Last Page: 513

Title: Trace Fossils Within Limestone Interbeds, Oak Grove Member, Carbondale Formation (Pennsylvanian, Desmoesian), Northwestern Illinois: ABSTRACT

Author(s): Glen K. Merrill

Article Type: Meeting abstract


Most trace fossils have been described from terrigenous rocks, commonly sandstones or sandstone/shale interbeds. The prevailing opinion appears to be that carbonate rocks rarely contain trace fossils or that ichnofossils are more difficult to study in them. Neither is true. Methodology is especially important in these studies, and the peculiarities of carbonates make them particularly rewarding subjects for trace fossil analysis.

At Wolf Covered Bridge in Knox County, Illinois, the Desmoinesian Oak Grove Member is more than 5 m (16 ft) thick and consists mostly of shale with several thin carbonate (limestone and siderite) interbeds. The two thickest of these carbonates are a lower "gray septarian" (or "Marginifera") limestone about 0.2 m (8 in.) thick, and about 1.5 m (5 ft) higher a 0.3 m (1 ft) thick "Linoproductus" limestone. Both are sparse to packed mixed biomicrites with diverse and abundant fossils. The depositional environments of these limestones were similar: nearshore, quiet, delta-influenced, somewhat brackish, shallow water deposits interpreted by Merrill (in 1975) to have formed in water less than 20 m (66 ft) deep.

Limestones were sliced perpendicular to bedding in the normal fashion and large slabs were also cut parallel to bedding with a wire saw. Serial sections cut perpendicular to bedding were photographed by X-radiography permitting three-dimensional reconstruction of some burrows. Large slabs cut parallel to bedding were etched and acetate peels prepared in the convention manner, but of unconventional size (some more than 1.0 m, 39 in., long). The polished surfaces were later gridded, coated, and the distribution of body and trace fossils mapped both megascopically and microscopically from the peels.

The level of bioturbation is exceedingly high, especially in the "Marginifera" limestone. Several generations of truncating trace makers are evident. Recognizable ichnogenera include a spectacular Rhizocorallium 40 cm (16 in.) long with waves of spreite outlined by calcitornellid (pseudopthalmid) foraminifers, numerous Chondrites up to 10 cm (4 in.) high, and common Planolites. Lithologic differences among burrow types are striking and many vague, relict, earlier generations of traces remain, traced primarily by allochem distribution. Substrate stabilities differed between the pair of limestones and bearing strength was probably a major factor controlling community structure. There are suggestions of "ghost biota" and lack of significant compaction of the micrite in the lower carbonat interbed.

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