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Description
The Mountain Springs Formation (Ordovician-Devonian) was deposited on the cratonic platform adjacent to the Cordilleran miogeocline. Three sections of the Mountain Springs Formation were measured in southwestern Clark County, Nevada. The Mountain Springs crops out in a northeast-southwest trending belt parallel to regional depositional strike. The three sections were chosen to examine the inner, middle, and outer portions of this belt to study the craton to miogeocline transition. The sections were examined for conodonts and lithology in order to determine the age and depositional environments of the formation. Four hundred three conodonts were retrieved from the westernmost section of the study area. No conodonts were recovered from the other two sections. The conodonts indicate that in the western end of the study area, the Mountain Springs Formation is time-stratigraphically equivalent to the following miogeoclinal formations: (1) the Lower to lower Middle Ordovician Pogonip Group, (2) the uppermost part of the Eureka Quartzite, (3) the Upper Ordovician part of the Ely Springs Dolomite, and (4) the Early Devonian Lone Mountain Dolomite (Nevada) and Hidden Valley Dolomite (California). Most of Middle Ordovician time and all of Silurian time are represented by disconformities. Lithologic correlation was used to establish four informal members. Member 1 was deposited on a gently west-dipping ramp during Early and early Middle Ordovician time. Intertidal mudflat and oolite shoal deposits occur in the central portion of the study area, while subtidal platform and tidal channel deposits characterize the west side of the study area. Member 2 is found only on the west side of the study area and is late Medial and Late Ordovician in age. It contains a progradational sequence of restricted marine deposits capped by calclithite conglomerate. This conglomerate deposit indicates moderate relief between the central and western portion of the study area. Clasts were eroded from the Mountain Springs Formation near Section MS7 and shed downhill to the west, where they were deposited near Section MS6. Member 3 also is restricted to the western side of the study area where it forms a thin interval of Upper Ordovician open marine subtidal deposits. Member 4 occurs at all three sections and was deposited during the Early Devonian when at least four minor sea level fluctuations left subequal thicknesses of intertidal and subtidal deposits separated by solution contacts and breccias.
