An Invertebrate Assemblage From the "Modelo" Formation of Reynier Canyon, Los Angeles, California. Geology and Origin of Talc Deposits of Eastern California

Abstract

An Invertebrate Assemblage From the Modelo Formation of Reynier Canyon, Los Angeles, California: An upper Miocene marine molluscan fauna occurs in basal Modelo sandstones that flank Reynier Canyon of the southeastern portion of the Ventura basin, Los Angeles County, California. Here two outliers of these beds are nonconformable on continental siltstones and tuffs of the Mint Canyon formation. The Modelo sandstones grade upward into punky diatomaceous shales also of the Modelo sequence, that contain Anadara cf. obispoana and a foraminiferal fauna reported to be Mohnian in age. The shales of one outlier are unconformably overlain by other marine sediments. The upper Miocene age of the basal Modelo beds is indicated by the presence of Clementia cf. martini, Dosinia arnoldi, Lyropecten estrellanus ss., Spisula albaria, Tivela diabloensis, and a large Ostrea. Approximately half of the forms in the Reynier Canyon Modelo fauna also occur in the Elamere Canyon fauna; these include Laevicardium centifilosum, L. quadragenarium var. fernandoense, Lucina nuttallii, Cancellaria elsmerensis, C. hemphilli, C. tritonidea, Surculites remondii, Murithais eldridgei, Nuculana taphria, and Turritella cooperi. The Elmsmere Canyon species Patinopecten lohri, Astrodapsis fernandoensis, and Dendraster sp. are missing from the Modelo fauna. Geology and Origin of Talc Deposits of Eastern California: A 200-mile northwest-trending belt in eastern California contains more than 100 talc-bearing localities. The belt is divisible into three districts; each with talc deposits in a distinctive terrane. In the southernmost district, near Silver Lake, tremolitic talc deposits have formed in highly metamorphosed Archean (?) sedimentary rocks extensively invaded by lamprophyre and granitic rocks. The development of talc-tremolite rock probably involved the extensive introduction of MgO and SiO2 to silica poor dolomite in a complex, multistage history. The MgO may have been released in the granitization of high-magnesian sediments. In the Southern Death Valley-Kingston Range region tremolitic talc deposits occur at or near the borders of a diabase sill intruded near the base of the lowermost carbonate unit in the Algonkian Pahrump series. The deposits have generally altered from dolomite, both siliceous and silica-poor. Other alteration rocks rich in alkali feldspar are associated with the talc deposits and border diabase bodies higher in the member. MgO and SiO2 have been introduced to form the talc bodies; SiO2, Al2O3, K2O, and probably Na2O to form the feldspathic rocks. Diabase magma was probably the source of most of the additive material, but some may have been derived from connate water. In the Inyo Range area tremolite-poor talc deposits have formed as alterations of Paleozoic dolomite and quartzite, and of Mesozoic granitic intrusives. The White Eagle deposit shows all three types of alteration, but has formed largely as a replacement of adamellite. Feldspars, quartz and ferromagnesian minerals were decomposed; the talc alteration followed an advance wave of albitization. The additive MgO probably was leached from dolomites at depth.