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At least 937 records · Page 52Linked to original sources
USGS1952

Stratigraphy of the outcropping formations in southern Florida

The geology of southern Florida, except for the Pliocene deposits, is not widely known among geologists and paleontologists. The popular concept is that there are very few outcrops in this part of the State. It is true that there are very few natural exposures, but the dredging of rivers and the construction of canals have made many exposures. The Tamiami formation is the only formation of Miocene age exposed at the surface that is recognized in southwestern Florida. The Hawthorn formation and the older Tertiary formations have been recognized in samples from wells in southern Florida.

Florida↗
USGS2002

Lithofacies and stratigraphy of the Lisburne and Etivluk groups in the Lisburne 1 well and adjacent outcrops

The Lisburne 1 well in the thrust belt of the central Brooks Range penetrated 17,000 ft of imbricated, chiefly Ellesmerian sequence strata in the Endicott Mountains allochthon. Five thrust repeats of the Lisburne Group (Carboniferous) and overlying Etivluk Group (Permian-Jurassic) were drilled. Lithofacies analyses of >350 thin sections of cores and cuttings, and biostratigraphy based on foraminifers and conodonts, allow detailed correlation with coeval units in adjacent outcrops and provide data on the depositional setting and reservoir and source rock potential of these strata. The late Early- Late Mississippian (Osagean-Chesterian) Lisburne Group consists mainly of skeletal wackestone to grainstone, locally completely dolomitized. An interval of abundant glauconite and detrital quartz in the lower Lisburne may mark a sequence-bounding unconformity. Dolostone in the upper part of the unit has maximum porosities of 10-13% and common residual hydrocarbons. The uppermost Lisburne is thinly interbedded mudstone, chert, and shale that are locally dolomitic, phosphatic, spiculitic, and organic-rich; conodonts from this interval in outcrop represent an outer shelf to slope biofacies. The Etivluk Group here encompasses the Siksikpuk and Otuk Formations. The Siksikpuk is mainly varicolored shale and radiolarian chert, with a basal interval of glauconitic, pyritic sandstone. Phosphatic and organic-rich shale, radiolarian chert, and pelecypod coquinas make up the Otuk. Outcrop and subsurface data indicate that the Lisburne Group in this area accumulated near the seaward margin of a shallow-water carbonate platform that drowned during the Late Mississippian; outer shelf or deeper conditions predominated throughout deposition of the upper Lisburne and the Etivluk Group.

AAPG Bulletin↗
USGS2010

Stratigraphy, age, and depositional setting of the Miocene Barstow Formation at Harvard Hill, central Mojave Desert, California

New detailed geologic mapping and geochronology of the Barstow Formation at Harvard Hill, 30 km east of Barstow, CA, help to constrain Miocene paleogeography and tectonics of the central Mojave Desert. A northern strand of the Quaternary ENE-striking, sinistral Manix fault divides the Barstow Formation at Harvard Hill into two distinct lithologic assemblages. Strata north of the fault consist of: a green rhyolitic tuff, informally named the Shamrock tuff; lacustrine sandstone; partially silicified thin-bedded to massive limestone; and alluvial sandstone to pebble conglomerate. Strata south of the fault consist of: lacustrine siltstone and sandstone; a rhyolitic tuff dated at 19.1 Ma (U-Pb); rock-avalanche breccia deposits; partially silicified well-bedded to massive limestone; and alluvial sandstone and conglomerate. Our U-Pb zircon dating of the Shamrock tuff by SHRIMP-RG yields a peak probability age of 18.7 ± 0.1 Ma. Distinctive outcrop characteristics, mineralogy, remanent magnetization, and zircon geochemistry (Th/U) suggest that the Shamrock tuff represents a lacustrine facies of the regionally extensive Peach Spring Tuff (PST). Here we compare zircon age and geochemical analyses from the Shamrock tuff with those of the PST at Stoddard Wash and provide new insight into the age of zircon crystallization in the PST rhyolite. Results of our field studies show that Miocene strata at Harvard Hill mostly accumulated in a lacustrine environment, although depositional environments varied from a relatively deep lake to a very shallow lake or even onshore setting. Rock-avalanche breccias and alluvial deposits near the base of the exposed section indicate proximity to a steep basin margin and detrital studies suggest a southern source for coarse-grained deposits; therefore, we may infer a southern basin-margin setting at Harvard Hill during the early Miocene. Our geochronology demonstrates that deposition of the Barstow Formation at Harvard Hill extended from before ~19.1 Ma until well after ~18.7 Ma, similar to timing of Barstow Formation lake deposition in the Calico Mountains but at least 3 million years older than comparable lacustrine facies in the Mud Hills type section. These observations are consistent with either of two paleogeographic models: westward transgression of lacustrine environments within a single large basin, or sequential development of geographically distinct eastern and western sub-basins.

California↗
USGS2016

Linking the Central and Southern Appalachian Blue Ridge: What We Know and Don’t Know about Stratigraphy, Structure, Tectonism, and Regional Correlation in the Eastern Blue Ridge of Virginia

The transition from Neoproterozoic Lynchburg Group rocks on the eastern limb of the para-autochthonous Blue Ridge anticlinorium in central Virginia to the fault-bounded Ashe Formation and Alligator Back Formation in southern Virginia has been a source of intense debate and speculation for decades. There are fundamental differences in the tectonogenetic interpretation for these rock packages, despite many similarities in lithology. This problem is compounded by insufficient detailed mapping (1:24,000-scale) in critical key areas of this transition. Lynchburg Group rocks unconformably overlie Mesoproterozoic meta-igneous rocks and underlie Catoctin greenstone on the east limb of the anticlinorium in central Virginia. In southern Virginia, similar metasedimentary lithologies – metasandstone (meta-feldspathic greywacke, meta-quartz arenite, meta-quartz wacke), graphitic schist, and pebble metaconglomerate – and mafic to ultramafic rocks of the Ashe Formation and Alligator Back Formation are faulted onto Mesoproterozoic meta-igneous rocks along the Gossan Lead and Red Valley faults. Internal to the eastern Blue Ridge of southern Virginia, the Rock Castle Creek fault separates Ashe Formation, consisting of Lynchburg-like lithologies, from polydeformed Alligator Back Formation rocks, which crop out in the core of the Ararat River synclinorium. Regional reconnaissance suggests Ashe Formation rocks re-emerge on the eastern limb of the synclinorium. Fundamental and conflicting differences in tectonogenetic models for these rocks compound the problem. The Neoproterozoic Lynchburg Group has long been thought to represent Laurentian margin rift-related rocks, with intrusive mantle-derived dikes and sills of mafic and ultramafic character. In contrast, the Ashe Formation and Alligator Back Formation in southern Virginia and northwestern North Carolina are possibly in part younger, and interpreted to be part of a distal margin accretionary wedge with entrained and tectonically emplaced dismembered ophiolite fragments of mafic and ultramafic rocks. Only detailed mapping in critical areas, coupled with new and emerging geochemical and geochronologic analyses will solve the persistent questions about the various units.

Book chapter↗
USGS2015

Early Permian conodont fauna and stratigraphy of the Garden Valley Formation, Eureka County, Nevada

The lower part of the Garden Valley Formation yields two distinct conodont faunas. One of late Asselian age dominated by Mesogondolella and Streptognathodus and one of Artinskian age dominated by Sweetognathus with Mesogondolella. The Asselian fauna contains the same species as those found in the type area of the Asselian in the southern Urals including Mesogondolella dentiseparata, described for the first time outside of the Urals. Apparatuses for Sweetognathus whitei, Diplognathodus stevensi, and Idioprioniodus sp. are described. The Garden Valley Formation represents a marine pro-delta basin and platform, and marine and shore fan delta complex deposition. The fan-delta complex was most likely deposited from late Artinskian to late Wordian. The Garden Valley Formation records tremendous swings in depositional setting from shallow-water to basin to shore.

Nevada↗
USGS1981

Subdivision and regional stratigraphy of the pre-Punta Gorda rocks (lowermost cretaceous-jurassic?) in South Florida

In recent years several wells have been drilled in the South Florida Basin through carbonate and evaporite sequences to depths as much as 5,300 ft below the Punta Gorda Anhydrite. The deepest well penetrated igneous basement rocks to a total depth of 18, 670 ft. Correlation of anhydrite beds below the Punta Gorda has revealed several thick anhydrite units (200 to 400 ft) with regional persistence. The pre-Punta Gorda section is subdivided into four easily identifiable units listed in order of increasing age — Lehigh Acres (lowermost Comanchean), Pumpkin Bay (upper Coahuilan), Bone Island (lower Coahuilan), and Wood River (Jurassic?) Formations, all newly named in this report. In addition, the Lehigh Acres is divided into the West Felda Shale (base), Twelve Mile, and Able Members which are also named and defined in this report. Geochemical evidence indicates that the Lehigh Acres unit and the upper part of the Pumpkin Bay unit contain the most likely source beds for petroleum. Only two production tests have been carried out in the basin in strata below the oil-productive Sunniland Limestone. One was through casing in a Wood River dolomite zone. It reportedly produced water and some gas. The other was a drill stem test in an upper Pumpkin Bay dolomite zone which produced only water. In the Gulf Florida State Lease 826Y (Permit No. 275), a moderately porous, 350-ft-thick Pumpkin Bay dolomite zone was observed. As this well is west of the axis of the basin, better reservoir conditions presumably exist on the West Florida shelf than onshore.

Florida↗