John C. Holden

LEAD-ALPHA DATES FOR SOME BASEMENT ROCKS OF SOUTHWESTERN CALIFORNIA

Lead-alpha measurements for zircon and monazite from Woodson Mountain Granodiorite, Ramona and Cuyamaca Peak quadrangles, give ages of about 110 m.y. Bonsall Tonalite, Bonsall and Cuyamaca Peak quadrangles, gives ages of about 120 m.y. Green Valley Tonalite, Escondido and Cuyamaca Peak quadrangles, gives ages of 130 m.y. Prebatholithic volcanic rocks from the Santiago Peak and Escondido quadrangles give ages of about 150 m.y. Granite and granodierite from the Palm Desert quadrangle give ages of 110 and 105 m.y. A quartz diorite from the Superstition Hills, Plaster City quadrangle, indicates 155 m.y. Detrital zircon in the prebatholithic Bedford Canyon Formation, Corona South quadrangle, and Julian Schist, Santa Ysabel quadrangle, give composite ages of 750–1000 m.y. A metasedimentary migmatite gneiss from the Orocopia Mountains, Canyon Spring quadrangle, yields a zircon age of 2400 m.y.

EARTH AND MOON: TECTONICALLY CONTRASTING REALMS

Introduction Any comparison of lunar and terrestrial structures is a study in contrasts. The basic difference is sometimes cited as being the lack of a lunar atmosphere and hydrosphere so that erosion is essentially nil and no sedimentary deposits a re formed. Also, the moon’s low gravitational field means less effective isostatic adjustments so tha t high topographic relief is allowed to persist. While these factors preserve the crispness of lunar physiography, they also show tha t the underlying geologic structures a re entirely different. An erosion-free earth, even if entirely stripped of i t s sedimentary cover, still would not resemble the moon. This paper is a n attempt to explain, in most general terms, why this remarkable tectonic contrast may exist.

Upper Cretaceous Ostracode Faunule from Carlsbad, California: ABSTRACT

A limited exposure of Upper Cretaceous marine siltstones and claystones near the coastal town of Carlsbad, southern California, contains a remarkably well preserved microfauna. Twenty-six species of ostracodes including twenty-three new species and one new trachyleberid genus occur in the upper part of the section. This faunule possesses distinct Cenozoic affinities expressed by the presence of the genera Trachyleberis, Actinocythereis, and Idiocythere, all of which previously have been reported from rocks no older than Lower Tertiary. Generic affinities with European faunas are also noted by new species of Idiocythere, previously reported from the Eocene of Germany only, and Isocythereis, previously from the Cretaceous of Germany only. Three species occur which also occur in Upper Cretaceous rocks of the U. S. Gulf Coast. These are Brachycythere darensis Swain, 1952, Krithe cushmani Alexander, 1929, and Cytheropteron coryelli Schmidt, 1948. End_of_Article - Last_Page 1771------------