Late Eocene Early Oligocene Research Articles

Geochronology of Tertiary igneous rocks, eastern Basin and Range Province, western Utah, eastern Nevada, and vicinity, U.S.A.

More than 100 new K-Ar dates for volcanic and intrusive rock specimens from the eastern Basin and Bange Province provide numerous tests of the dating technique based on comparisons between samples from the same and sequentially related units and between mineral separates from the same or related samples. Using single samples, Tertiary volcanic units may be dated with a precision (σ) of 4 per cent. Between-sample variance is larger than analytical error. With multiple samples the precision of age determination may reach 1–2 per cent. In order of decreasing suitability (increasing deviation from average dates for multiply dated units) for dating volcanics are: biotite, sanidine, hornblende, plagioclase, pyroxene and whole rock samples, but none of these materials shows serious Ar deficiencies or excesses. Unweathered but hydrated glass gives dates consistently about 10 per cent low. Biotite and Muscovite are suitable for dating plutons; K-feldspar shows small Ar deficiencies hornblende appears to be acceptable, but pyroxene separates may contain extraneous Ar, and are thus considered unreliable. All available dates show a consistent regional pattern. Scattered igneous centers existed in late Eocene-early Oligocene time in the Wasatch region of Utah, in northern Nevada and probably at Gold Hill, Utah. Intense igneous activity occurred in and near east-central Nevada during the Oligocene; activity shifted towards the eastern and southern margins of the Great Basin during Miocene and Pliocene time and gradually decreased in intensity so that at present only sporadic eruption of mafic lavas occurs. In individual areas, dates on intrusive and extrusive igneous rocks are in close agreement and activity is notably episodic (within a period of about 5 million years). Eruption of silicic volcanics is quickly followed by mafic volcanism that marks the end of one of the short episodes. Regionally the short episodes are smeared out into a broad pulse of Tertiary igneous activity between 40 and 5 million years ago.

Upper Cretaceous-Cenozoic Paleobathymetric Cycles, Eastern Panama and Northern Colombia: ABSTRACT

ABSTRACT Analysis of planktonic microfossils and benthic foraminifera indicates one deep-water depositional cycle in the Upper Cretaceous of northern Colombia and two major deep-water sequences in the Cenozoic of both northern Colombia and eastern Panama. Only slight evidence of an Upper Cretaceous deep-water cycle was found in eastern Panama. The Upper Cretaceous deep-water cycle in the radiolarian-rich Campanian sequence of northern Colombia is characterized by a Dictyomitra multicostata radiolarian assemblage. Deep-water or abyssal depths of the next younger cycle (Upper Paleocene-Lower Eocene) are suggested by a Pleurostomella-Nuttallides fauna in combination with a rich radiolarian assemblage. A third abyssal sequence or cycle, in Middle Oligocene-to-Lower Miocene strata, is indicated by a Melonis pompilioides fauna together with a rich radiolarian assemblage. The shallowest-water facies, separating the deep-water cycles, represent mostly neritic or upper-bathyal depths. These are characteristic of the basal Paleocene, the Upper Eocene-Lower Oligocene, and the Upper Miocene through Quaternary sequences of eastern Panama and northern Colombia. Locally, unconformities and/or nonmarine beds may represent these geologic ages. In contrast to the deep-water cycles of eastern Panama, the sections of the Gatun Lake area, to the west of the Rio Limon fault, show relatively shallow-water marine facies (neritic to upper-bathyal depths at the most). These shallow marine facies occur in the Eocene, in the Upper Oligocene, and in the Upper Miocene-Pliocene sections. They are separated by either paralic beds or unconformities. Faults such as the Rio Limon fault separate tectonic blocks that have contrasting stratigraphic and depositional records throughout most of the Cenozic. In eastern Panama and northern Colombia, the shallowest-water horizons of each paleobathymetric cycle may represent times conducive to the migration of land faunas. These times are: latest Cretaceous-earliest Paleocene; late Eocene-early Oligocene; Pliocene and Quaternary.