Bol. Soc. Geol. Mexicana, Tomo XLI, Nos. 1 y 2, 1980
Stratigraphy, depositional enviroments and foraminifera of the Miocene Tortugas Formation, Baja California Sur, Mexico
http://dx.doi.org/10.18268/BSGM1980v41n1a3
Javier Helenes Escamilla*
*Department of Geology, Stanford University, Stanford, California, U.S.A.
Abstract
The Lower through Upper Miocene Tortugas Formation is widely exposed on the Vizcaino Peninsula of Baja California Sur, Mexico, and provides a clear record of the evolution of a southern continental borderland basin. The diatomaceous sediments of this formation are similar in many respects to the correlative Miocene Monterey Shale of Alta California.
The integrated study of foraminifera, diatoms, and sedimentologic characteristics through a 400 m thick section of the Tortugas Formation located in a faulted block, east of Punta Quebrada, has allowed this unit to be subdivided into four members reflecting variations in paleobathymetry, depositional environments, and tectonic and/or climatic events. The lowermost member 1 consists of 60 m of Lower to Middle Miocene (Saucesian) silty mudstones containing middle bathyal foraminifera. Member 2 is composed of 65 m of interbedded spicular sandstones, tuffaceous sands, and silty mudstones, representing submarine fan deposition and erosion of previously deposited sediments on the basin floor during Middle Miocene time (Relizian-Luisian). Member 2 also reflects the presence of volcanic activity and an uplift event during that periodo Members 3 and 4 are composed of 300 m of alternating porcellanites (mainly found in the lower 50 m) and diatomaceous mudstones representing deposition in an anoxic, middle bathyal, silled basin during late Middle and early Late Miocene (Mohnian) time.
Study of a second section through the Tortugas Formation located east of the airfield at Bahia Tortugas allowed the division of this sequence into 2 members. The basal member A consists of 45 m of lithic sandstones containing pelletal phosphorite and neritic mollusks of Early to Middle Miocene age (upper Saucesian to Relizian?) and representing a neritic facies correlative with the upper part of member 1 and with member 2 representing bathyal conditions in the Punta Quebrada section to the north. Member B consists of 95 m of silty, diatomaceous shales containing foraminifera indicative of a silled anoxic basin environment similar to the environment recognized in members 3 and 4 of the Punta Quebrada section.
In summary, three stages in the geologic evolution of the Tortugas basin are recognized: 1) Early Miocene (late Saucesian) rapid subsidence from subaerial to a middle bathyal depth of a low rate of sediment accumulation; 2) A Middle Miocene (Relizian-Luisian) uplifting event that triggered deposition of turbidites in the deeper part of the basin, and 3) Renewed subsidence and equal rate of sedimentation in the Late Miocene (early Mohnian) resulted in a low oxygen environment at upper bathyal depths.
Uplift of the Tortugas basin sequence occured during the Miocene-Pliocene interval with further structural deformation of this area during Pleistocene time.
