Abstract: Devonian and Late Paleozoic Carbonate Buildups of Idaho

ISAACSON, PETER E., Department of Geology, University of Idaho, Moscow, ID 83844-3022

Devonian Buildup

An Upper Devonian (upper Frasnian), coral-dominated carbonate buildup (40m) occurs in the upper part of the “dark dolomite member” of the Jefferson Formation at Grandview Canyon (Custer County) in east-central Idaho (Isaacson and Dorobek, 1989). The buildup formed above a thick sequence of slope to deep ramp deposits and therefore reflects regional shallowing of deep water environments. All rock units are dolomitized and consist of a range of lithologies, including floatstone, fossiliferous wackestone and packstone, and rare bindstone. The buildup shows four definitive biofacies, which may reflect some of the successional stages of Walker and Alberstadt (1975). Establishment of the buildup is on an irregular hardground surface. Biofacies A is laterally persistent and relatively thin (4-7m). It is characterized by abundant hemispherical growth forms of the colonial rugose coral Peneckiella and bulbous but small stromatoporoids. Biofacies B (up to 10-15m) is the most distinctive biogenic component of the buildup. “Thamnopora” is the dominant faunal constituent and forms rock generally known as “spaghetti stone” in other Late Devonian rocks in the Cordillera. Some colonies are more-or-less in situ, although many are re-oriented. Biofacies C (10-12m) reflects interaction with wavebase and is dominated by floatstone. Stromatoporoids of variable growth forms, including Amphipora occur with fragments of other organisms, including disarticulated brachiopods, solitary rugose corals and pelmatozoan columnals. Biofacies D is composed of intact, hemispherical colonies of Syringopora in bioturbated dolo-mudstones and wackestones. This stage reflects a deepening or transgressive event, which is abruptly terminated by an erosional surface of up to 2m relief.

Pennsylvanian/Permian Buildups

Contained within a thick sequence of Pennsylvanian-Permian carbonates in the Arco Hills and southern Lemhi Range, south-central Idaho, are five recurrent or cyclic algal buildups (Breuninger, Canter and Isaacson, 1988). The Juniper Gulch Member of the Snaky Canyon Formation is approximately 600m thick and contains four depositional facies, 1) open circulation shelf margin, 2) hydrozoan and phylloid algal mound-dominated carbonate buildup, 3) backmound to restricted marine, and 4) open circulation to slightly restricted platform facies. Within the phylloid algal mounds are several microfacies, including lime mud-rich bafflestone, diversely fossiliferous packstone and grainstone, bryozoan lime floatstone, and phylloid algae and hydrozoan (Palaeoaplysina) lime bindstone. Collectively, these microfacies represent sedimentation on or in proximity to a phylloid algae and Palaeoaplysina-dominated carbonate buildup.

These mound-shaped buildups appear to be composed principally of the phylloid codiacean algae Eugonophyllum(?), associated skeletal debris, lime mud, and sparry calcite. Growth habit of the phylloid algae was upright, reached a height of 6cm (Wray, 1977), consisted of broad, that “leaves” which baffled sediment (Toomey and Babcock, 1983), grew rapidly, and existed largely at the exclusion of other organisms. Therefore, the algal community was of low diversity, although the organisms' umbrella structures provided habitats for some invertebrate colonial and shelly faunas. Autogenic and allogenic successional stages are recognized within the buildups. Colonial rugose corals effected a stabilzation stage. When the algal communities of the diversification stagereached wave base, on account of their rapid upward growth, cross-bedded oolitic grainstone and occasional cross-bedded dolomite shoals ensued. Supratidal to high intertidal platform sedimentation is represented by dolomitic Palaeoaplysina bindstone, algal mat bindstone, and vuggy dolomite. Five vertical sequences o f buildup development, each terminated by intertidal, supratidal, or erosional events, are seen in the Juniper Gulch Member in the North Howe stratigraphic section of the southern Lost River Range. Shelf subsidence, overprinted by eustatic sealevel changes, provided ideal environmental positioning for the successive buildups, with attendant backmound and restricted facies development to the east.

AAPG Search and Discovery Article #90937©1998 AAPG Annual Convention and Exhibition, Salt Lake City, Utah