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Heironimus_Thurman_L_1981_Plate3.pdf

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  • During Early and Middle Devonian time, the main part of the Death Valley area was characterized by shallow marine shelf position. In the extreme western part, deposition took place in carbonate slope environments. Early and Middle Devonian eastern platform dolomites, represented by uppermost Hidden Valley Dolomite and lower Lost Burro Formation, have been divided into six units in ascending order based on lithology, depositional environment, and age. The Lochkovian depositional setting was that of southeast to northwest transition from a broad inner platform to a narrow outer shelf basin. Unit 1 intertidal to very shallow subtidal sediments were deposited to the east, whereas a skeletal carbonate bank or reef complex fringing the platform edge supplied bioclastic material to extreme slope areas to the west. Late Lochkovian regression exposed extreme southeast thesis platform areas to erosion. A depositional hiatus marks the top of farther seaward Unit 1 platform areas in the northern Panamint Range. Unit 2 fossiliferous sediments were widely deposited throughout the thesis area during early Pragian transgression. Unit 2 correlative Sevy-like deposits in southeast areas at Bat Mountain and the Nopah Range mark the landward extent of initial marine onlap during this time. With increased input of clastic material into the normal marine environment, the Unit 2 lithotope was gradually replaced by Unit 3 silty and cherty argillaceous dolomites. The lower part of Unit 3 was deposited during continued marine transgression. The upper part reflects shallowing upward conditions and initiation of late Early Devonian regression. To the west, slope areas contain cherty argillaceous limestones correlative to Unit 3 and, similarly, indicate shallowing conditions. A beach-barrier bar complex, represented by sandy Unit 4 rocks, prograded seaward over nearshore Unit 3 sediments in response to marine offlap in late Early Devonian time. Unit 4 reflects an increased clastic sediment supply derived from erosion of cratonward strata, as well as marking regression. The seaward prograding sandy environment was followed and progressively overstepped by eastern intertidal-supratidal deposits, represented by Unit S coarse crystalline dolomite. Thinness and gradational lower contact of this lithology suggest that the transgressive part of coarse crystalline dolomite deposition seen in central Nevada may not be present in southeast California. Deposition of Unit 6 intertidal to shallow subtidal sediments during early Middle Devonian time records a gradual submergence of inner shelf regions by initial transgression. Rocks overlying Unit 6 in the thesis area reflect the larger transgressive pulse of the Taghanic onlap. Two distinct dolomite types are recognizable in the thesis area. Primary dolomite, represented by aphanitic, thinly laminated dolomite at the Nopah Range and Bat Mountain, was formed by penecontemporaneous replacement of calcareous sediment essentially at time of deposition. This dolomite type preserved microcrystalline textures, and fine sedimentary structures. The second dolomite type, eogenetic secondary dolomite, represents postdepositional near-surface replacement of limy sediments by slow growth and coalescence of dolomite crystals. This dolomite type characterizes the bulk of the carbonate interval studied. A favored mechanism for eogenetic secondary dolomite formation in the thesis area is that of mixing zone dolomitization, whereby meteoric freshwater mixes with intrastratal sea water in a seaward extending subsurface wedge. This dolomitizing zone moved seaward with marine regression and landward during transgression, leaving behind a blanket of dolomitized strata. Coarse crystalline dolomite in Unit 5 was extensively subjected to this paleogeographic-related dolomitizing environment.
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