About This Item
Share This Item
The west side of Baja California between 30° and 32° North exhibits a terrane of deformed pre-batholithic eugeosynclinal rocks, abundant batholithic and smaller intrusive bodies, and a coastal zone of relatively undeformed post-batholithic sediments.
The pre-batholithic section of great but unknown thickness consists of basic and intermediate volcanic pyroclastics and flows, graywackes, and rare carbonates. It has been intensely folded and faulted, and near contacts with plutonic intrusives is mildly to strongly metamorphosed. A persistent limestone zone has been a useful stratigraphic unit in regional study. Numerous fossils taken from this zone and from other horizons indicate Albian age.
Plutonic rocks of considerable variety intrude the eugeosynclinal terrane but occur most commonly
away from the coast. Absolute age determinations by both Larsen and isotope dilution techniques have confirmed previous regional correlation of the batholith of Baja California with the Southern California batholith.
Relatively undeformed post-batholithic rocks along the Pacific Coast lie across beveled erosion surfaces on both pre-batholithic and plutonic rocks, clearly establishing the age relations. Abundant fossils indicate that the oldest exposed sediments in this sequence are of Maestrichtian age.
A close point of correlation has been established between the relative and absolute time scales. The absolute age of the intrusive rocks, 115(±10) × 106 years, falls in the interval of time bracketed by the Albian below and the Maestrichtian above. The chronological and geological relations of the Baja California rocks to those of adjacent regions are considered.
End_of_Article - Last_Page 216------------