Late Ordovician Paleogeography of Central Idaho and its Tectonic Implications

MEASURES, ELIZABETH A., University of Idaho, Moscow, ID

The Late Ordovician (Cincinnatian, Ashgill) paleogeography of central Idaho has been weakly constrained in the past. Previously, this area was treated as a simple extension of the miogeocline-eugeocline paleogeography defined from units in the Great Basin (Nevada and Utah). Detailed analysis of the Upper Ordovician Fish

Haven Dolomite enables a more refined paleogeographic interpretation to be made at this time.

The Fish Haven Dolomite (260 m or 800 ft, average thickness) of central Idaho is composed of 16 different carbonate facies which can be grouped into three sequences; facies within each sequence are depositionally related. The basal sequence is a blanket deposit composed of normal marine, shallow to deep subtidal, bioclastic dolowackestones. The western sequence, aerially restricted, occurring above the basal sequence, is composed of laminated dolomudstones and intraclastic dolorudstones of oxygen-poor, deep subtidal conditions. The eastern sequence, forming the bulk of the formation and overlying the basal sequence, is lithologically variable. It is dominated by normal marine, deep to shallow subtidal, bioturbated, bioclastic dolowackestones and oncolitic dolorudstones and capped by c oss-bedded dolograinstones.

Overall, the facies and sequences indicate that a carbonate ramp formed in central Idaho outboard of the craton and a hinge zone, within a subsiding area of the miogeocline. Shallow subtidal ramp deposits were probably deposited in approximately 30 m (100 ft), or less, of water depth. Deep subtidal, laminated ramp deposits were probably deposited in approximately 100 to 60 m (300 to 200 ft) of water depth. During this time interval, open ocean, anoxic bottom waters extended up into deep subtidal regions (60 m or 200 ft).

The outermost part of the ramp deepened upward as indicated by western over basal sequence. This portion of the ramp was also subject to downslope movement of outer ramp deposits. The remainder of the ramp, its central to inner parts, shallowed upward as indicated by eastern over basal sequence. The shallowing-upward and deepening-upward cycles were approximately coeval events along the ramp.

This information indicates that the Late Ordovician carbonate ramp underwent backstepping at its outermost portion resulting in drowning of the western ramp, and eventual migration of transitional facies deposits (Roberts Mountains Formation) over miogeoclinal deposits. Simultaneously, the remainder of the ramp shallowed-upward into shoals and beach deposits. Starved basin conditions existed in the eugeocline as only 22 m (70 ft) of the argillaceous Phi Kappa Formation was deposited. Tectonics played an active part in the deposition of the Fish Haven Dolomite in the miogeocline of central Idaho. The ultimate cause of the tectonism is not known at this time, but could be related to changes in the rate of sea-floor spreading, active structures within the continental margin, proximity of the Ordovician Klamath Mountains island arc Terrane, or unknown processes.

AAPG Search and Discovery Article #91009©1991 AAPG-SEPM-SEG-SPWLA Pacific Section Annual Meeting, Bakersfield, California, March 6-8, 1991 (2009)