Description
Carbonates of member A (Lower to lower Middle Ordovician) of the Mountain Springs Formation were deposited along the cratonic margin of the Cordilleran miogeocline. Deposition occurred in four major paleoenvironments: (1) uppermost intertidal mudflat, (2) upper to middle intertidal, (3) restricted lagoon, and (4) lower intertidal to shallow subtidal. The resultant depositional facies represent an onlapping succession of uppermost intertidal to shallow subtidal strata. Intermember and intramember subaerial solution features, and intrastratal and solution layers suggest humid paleoclimate during and immediately after deposition. The uppermost intertidal mudflat facies is dominated by dolomudstone, lime sand dolowackestone, and cryptalgal dolobindstone, and has subordinate intraclastic dolofloatstone. Most dolobindstone occurs as laminated, mat-like intervals; small columnar, digitate, domal, and polygonal(?) stromatolites are present locally. Most dolofloatstones are monolithologic. Planar laminae and beds are the dominant stratification types; lenticular beds are present locally. Rare to moderately common bioturbation is typically restricted to bedding plane surfaces. The upper to middle intertidal facies is dominated by lime sand dolowackestone, oolitic-peloidal dolowackestone, and monolithologic or polylithologic intraclastic dolofloatstone, and has subordinate dolomudstone and cryptalgal dolobindstone. Most dolobindstone occurs as thin, laminated, mat-like intervals; small domal and digitate stromatolites are present locally. Planar laminae and beds are the dominant stratification types. Subordinate wavy, lenticular, and flaser bedding, and bipolar foreset, ripple, and cross-laminae are present. Mild to intense bioturbation is rare to common. The restricted lagoon facies is dominated by cryptalgal dolobindstone, dolomudstone, and lime sand dolowackestone and has subordinate bioclastic dolowackestone, monolithologic or polylithologic intraclastic dolofloatstone. The bindstone occurs as thick, laterally extensive, laminated, mat-like intervals; distinct mound shaped build-ups are present locally. Bioclastic debris is dominated by transported pelmatozoan, brachiopod, and trilobite fragments. Bedding is weakly developed, with wavy to planar laminae and planar beds as the dominant stratification types. The intraclastic and bioclastic dolostones occur as discontinuous layers exhibiting scoured contacts with underlying strata. Bioturbation is rare to absent. The lower intertidal to shallow subtidal facies is dominated by lime sand dolowackestone, bioclastic dolowackestone, and polylithologic intraclastic dolofloatstone, and has subordinate dolomudstone and cryptalgal dolobindstone. Bioclastic debris is dominated by transported pelmatozoan, brachiopod, and trilobite fragments. Dolobindstone is present locally as oncoids and thrombolites. Planar beds are the dominant stratification type; lenticular bedding, and cross-laminae are present locally. Bioturbation is moderately common. Initially deposited as marine limestones, rocks of member A have undergone pervasive alteration to well-ordered dolomite. Dolomitization occurred in the shallow subsurface and resulted from mixing of meteoric and marine waters. Field and petrographic relationships indicate dolomite genesis took place after deposition, but prior to the Medial Devonian. Silicification has occured within member A. Chert, intercrystalline and interstitial silica cement, and horizons of silica replaced dolostone are present locally. Some chertification occurred prior to dolomitization.
