Abstract

The Monterrey Formation at San Hilario and San Juan de la Costa in Baja California Sur, Mexico, consists of predominantly fine-grained sediments that were deposited with almost no lateral variation during a slow marine transgression representing intertidal to outer shelf, low-energy environments. Deposition took place during late Oligocene to early Miocene times as evidenced by radiometric, diatom, and foraminiferal data.Stratigraphically, from bottom to top, the Monterrey section is composed of (1) a phosphaticvolcanic conglomerate unit, (2) a tuff unit having two diatomaceous facies at different stratigraphic levels as well as a phosphatic facies, and (3) a widely distributed siliceous shale unit having a calcareous facies and a phosphoritc facies. Maximum thickness of the Monterrey Formation at the San Hilario area is 103 m, being thinner toward the southeast whereas at the San Juan de la Costa area the total thickness is not known because the base of the Monterrey is unconformably overlain by Holocene beach deposits.Gradually decreasing volcanic activity from the Mexican mainland to the east of the Monterrey basin contributed most of the material that constitutes the lower half of the Monterrey; sporadic tuff beds in the upper half of the Monterrey and volcanic material in the phosphate beds indicate times of small increase in activity during deposition of the upper half.Pulses of cold and warm oceanic waters derived from the north and central Pacific characterized the Monterrey basin as evidenced by the occurrence of several species of diatoms as well as planktonic and benthonic foraminifera; the distribution of diatom beds also indicates that the water temperature was slightly warmer on the east side of the basin at San Juan de la Costa.The Monterrey basin was probably very large, extending over most of the actual Baja California Sur; that basin, after reaching its maximum extent, was partially limited on the west and south by Mesozoic granitic islands and on the east by a volcanic are migrating from the Mexican mainland toward the Monterrey basin area.

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