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USGS2024

Fault activity in the San Gabriel Mountains, southern California, USA: Insights from landscape morphometrics, erosion rates, and fault-slip rates

Many studies use landscape form to determine spatial patterns of tectonic deformation, and these are particularly effective when paired with independent measures of rock uplift and erosion. Here, we use morphometric analyses and 10 Be catchment-averaged erosion rates, together with reverse slip rates from the Sierra Madre−Cucamonga fault zone, to reveal patterns in uplift, erosion, and fault activity in the range front of the San Gabriel Mountains in southern California, USA. Our analysis tests two prevailing hypotheses: (1) the range front of the San Gabriel Mountains is at steady state, in which rock uplift balances erosion and topographic elevations are stable throughout time, and (2) that west-to-east increases in elevation, relief, erosion rate, and stream-channel steepness across the interior of the massif reflect a parallel reverse-slip rate gradient on the range-bounding Sierra Madre−Cucamonga fault zone. We show that although deviations from steady state occur, the range-front hillslopes and stream channels are typically both well-connected and adjusted to patterns in Quaternary uplift driven by motion on the range-front fault network. Accordingly, landscape morphometrics, 10 Be erosion rates, and model erosion rates effectively image spatial and temporal patterns in uplift. Interpreted jointly, these data reveal comparable peak slip rates on the Sierra Madre−Cucamonga fault zone and show that they do not monotonically increase from west to east. Thus, the eastward-increasing gradients developed within the interior of the massif are not solely related to reverse slip on the range-front faults. Evaluated on shorter length scales (<10 km), morphometric data corroborate earlier descriptions of the Sierra Madre−Cucamonga fault zone as multiple individual faults or fault sections, with slip rates tapering toward fault tips. We infer that these patterns imply the predominance of independent fault or fault section ruptures throughout the Quaternary, though data cannot rule out the possibility of large, connected Sierra Madre−Cucamonga fault zone ruptures. Deeper in the hanging wall of the Sierra Madre−Cucamonga fault zone, secondary faults accommodate range-front uplift. Motion on these faults may contribute to active uplift of the highest topography within the massif, in addition to partly reconciling differences between geologic and geodetic Sierra Madre−Cucamonga fault zone reverse-slip rates. This study provides a new, unified perspective on tectonics and landscape evolution in the San Gabriel Mountains.

California↗

USGS1968

Gneissic amphibolite at Las Palmas, Puerto Rico, and its significance in the early history of the greater antilles island arc

The basal complex of Puerto Rico consists principally of serpentinite, minor amounts of chert and spilite, and locally small blocks of amphibolite . A detailed structural and metamorphic study of a relatively large block of gneissic amphibolite at Las Palmas reveals that the rock has undergone repeated deformation and regional metamorphism prior to contact metamorphism by intrusive serpentinite. The first event recorded by the amphibolite is amphibolite facies regional metamorphism. Foliation and some hornblende lineation were formed at this time. An increase in the intensity of regional metamorphism then produced quartzo-feldspathic segregations from the host rock, and was accompanied by small-scale folding and development of several types of lineation oblique (o the early hornblende lineation. The metamorphic intensity then decreased slightly and tectonic movement radically decreased, as nonfoliated but metamorphosed mafic dikes cut the gneissic amphibolite . As the metamorphic grade decreased further, late-stage kink-bands were developed at high angles to the earlier small-scale folding. Later, the amphibolite was intruded by serpen-tinite, causing incipient low-grade contact metamorphism and some calcium metasomatism. H. H. Hess has maintained that Puerto Rico and other islands in the Greater Antilles rest directly on oceanic crust, and that the intrusive serpentinite is derived from the bottom layer of the crust. In the Las Palmas area, the amphibolite formed prior to intrusion of serpentinite, indicating a complex preserpentinite history . Three interpretations of the gneissic amphibolite are considered. It may represent: (1) metamorphosed gabbroic rocks genetically related to the serpentinite, (2) regionally metamorphosed mafic rocks that are part of the oceanic crust, or (3) older mafic rocks regionally metamorphosed as part of an extensive pre-Late Cretaceous orogeny. The evidence from Las Palmas is not conclusive, but when considered in the regional framework, either of the last two interpretations seems reasonable.

Puerto Rico↗

USGS1999

Window into the Caledonian orogen: Structure of the crust beneath the East Shetland platform, United Kingdom

Reprocessing and interpretation of commercial and deep seismic reflection data across the East Shetland platform and its North Sea margin provide a new view of crustal subbasement structure beneath a poorly known region of the British Caledonian orogen. The East Shetland platform, east of the Great Glen strike-slip fault system, is one of the few areas of the offshore British Caledonides that remained relatively insulated from the Mesozoic and later rifting that involved much of the area around the British Isles, thus providing an “acoustic window” into the deep structure of the orogen. Interpretation of the reflection data suggests that the crust beneath the platform retains a significant amount of its original Caledonian and older architecture. The upper to middle crust is typically poorly reflective except for individual prominent dipping reflectors with complex orientations that decrease in dip with depth and merge with a lower crustal layer of high reflectivity. The three-dimensional structural orientation of the reflectors beneath the East Shetland platform is at variance with Caledonian reflector trends observed elsewhere in the Caledonian orogen (e.g., north of the Scottish mainland), emphasizing the unique tectonic character of this part of the orogen. Upper to middle crustal reflectors are interpreted as Caledonian or older thrust surfaces that were possibly reactivated by Devonian extension associated with post-Caledonian orogenic collapse.

Geological Society of America Bulletin↗

USGS2014

A field trip guidebook to the type localities of Marland Billings' 1935 Paleozoic bedrock stratigraphy near Littleton, New Hampshire

Marland Billings' classic paper published in 1937 in the Geological Society of America Bulletin established a succession of six stratigraphic units in rocks of low metamorphic grade near Littleton, New Hampshire. The two youngest units are fossiliferous in the area, with ages established at the time as “middle” Silurian and Early Devonian. Billings and students mapped the same stratigraphic section in adjacent areas of progressively higher regional metamorphic grade. This work laid the foundation upon which a major part of subsequent work in New England has been directly or indirectly built. This guidebook was written for a field trip held in March 2013 to visit roadcuts that are as close as possible in March to the type localities or areas of Billings’ six-fold stratigraphic succession. Ten stops are in rocks of chlorite grade of Acadian(?) metamorphism; the final stop visits amphibolite of the Ammonoosuc Volcanics. Fieldwork by the authors over the past 20 years confirms Billings’ broad conclusions.

New Hampshire↗

USGS1953

Petrology of granophyre in diabase near Dillsburg, Pennsylvania

Small bodies of granophyre occur in the upper part of diabase bodies of Triassic age in southeastern Pennsylvania. One near Harrisburg was penetrated by a diamond-drill. Drill core specimens show a gradation from diabase to granophyre. New data include 10 chemical analyses, spectrographic determinations of trace elements, and the results of petrographic study of specimens from the drill core. The sequence, from diabase to granophyre, includes a chilled zone that represents an original magma of tholeiitic composition, normal diabase, pegmatitic facies of diabase, and granophyric diabase that is intermediate in composition and petrographic characteristics between diabase and granophyre, and finally granophyre. Alkalies and silica increase progressively from diabase to granophyre; iron increases to a maximum in transitional granophyric diabase, then decreases in the granophyre. It is concluded that crystal fractionation in a large sheetlike body of tholeiitic magma yielded a small amount of granophyre. Prior to complete solidification, a residual liquid rich in iron, alkalies, and silica accumulated locally in the upper part of the diabase sheet. In places volatile-rich iron-bearing solutions escaped into the overlying sedimentary rocks and deposited magnetite; the remaining liquid crystallized t o fine-grained granophyre.

Pennsylvania↗

USGS2014

Himalayan gneiss dome formation in the middle crust and exhumation by normal faulting: New geochronology of Gianbul dome, northwestern India

A general lack of consensus about the origin of Himalayan gneiss domes hinders accurate thermomechanical modeling of the orogen. To test whether doming resulted from tectonic contraction (e.g., thrust duplex formation, antiformal bending above a thrust ramp, etc.), channel flow, or via the buoyant rise of anatectic melts, this study investigates the depth and timing of doming processes for Gianbul dome in the western Himalaya. The dome is composed of Greater Himalayan Sequence migmatite, Paleozoic orthogneiss, and metasedimentary rock cut by multiple generations of leucogranite dikes. These rocks record a major penetrative D2 deformational event characterized by a domed foliation and associated NE-SW–trending stretching lineation, and they are flanked by the top-down-to-the-SW (normal-sense) Khanjar shear zone and the top-down-to-the-NE (normal sense) Zanskar shear zone (the western equivalent of the South Tibetan detachment system). Monazite U/Th-Pb geochronology records (1) Paleozoic emplacement of the Kade orthogneiss and associated granite dikes; (2) prograde Barrovian metamorphism from 37 to 33 Ma; (3) doming driven by upper-crustal extension and positive buoyancy of decompression melts between 26 and 22 Ma; and (4) the injection of anatectic melts into the upper levels of the dome—neutralizing the effects of melt buoyancy and potentially adding strength to the host rock—by ca. 22.6 Ma on the southwestern flank and ca. 21 Ma on the northeastern flank. As shown by a northeastward decrease in 40 Ar/ 39 Ar muscovite dates from 22.4 to 20.2 Ma, ductile normal-sense displacement within the Zanskar shear zone ended by ca. 22 Ma, after which the Gianbul dome was exhumed as part of a rigid footwall block below the brittle Zanskar normal fault, tilting an estimated 5°–10°SW into its present orientation.

Gianbul dome↗

USGS1973

Reinterpretation of the boundary between the Cosumnes and Logtown Ridge Formations, Amador County, California

Recent detailed geologic mapping in the Sierran foothills reveals that rocks previously included in the Jurassic Amador Group must be redefined. The term “Amador Group” was applied by Taliaferro and Clark to a section of epiclastic metasedimentary rocks (the Cosumnes Formation) and the seemingly conformable overlying metavolcanic rocks (the Logtown Ridge Formation). New structural and stratigraphic evidence indicates that at their type localities on the banks of the Cosumnes River the boundary between the two formations should be relocated about 610 m downsection from the position shown by Clark. This change removes all known paleontological control on the age of the Cosumnes Formation. Structural relations show that the type Cosumnes and Logtown Ridge Formations are in fault contact at the Cosumnes River. Rocks of the Cosumnes Formation are now grouped with a complex unit of megabreccia that includes other strata previously termed the “western belt” of the Calaveras Formation. The megabreccia formed, at least partly, sometime between late Paleozoic and Late Jurassic times, but rocks in the megabreccia, including the Cosumnes Formation, could be older than late Paleozoic. The term “Amador Group” is herein abandoned.

California↗

USGS1966

Jura tectonics as a décollement

For many years the structure of the Jura Mountains was interpreted as a décollement whose origin was related to the Alps; in recent years, however, this mode of origin has been questioned. Most of the alternative explanations recognize a décollement to some extent, but attribute it to movement of the basement beneath. Surface and subsurface data are here reviewed to show that the Jura deformation was produced in a gliding sheet, in which the forces of gravity and inertia were generated within the total moving mass. Features of the folded Jura which support the décollement hypothesis are: (1) Nowhere are rocks older than Middle Triassic exposed, which strongly suggests that the folding does not extend to the older rocks. (2) Subsurface data in the Lons-le-Saunier region clearly show that the external border of the Jura has moved northwestward over the eastern margin of the Bresse Basin. (3) Lower Jurassic rocks rest on Upper Jurassic along a horizontal fault 1234 m deep in the Risoux well near the middle of the Jura. (4) The tabular areas, with their absence of folds, are expectable in a décollement. (5) High-angle tear faults, interpreted as not extending into the basement, are normal features of a décollement sheet. A continuous décollement around the southwestern end of the Swiss Plain can reasonably be inferred, connecting the internal Jura, the Salève, and the Subalpine folds as part of the décollement mass. Elsewhere, the internal border of décollement extends southeastward into the Molasse basin for an unknown distance and probably underlies the entire basin; if it does, a causal relation to the Alps is indicated. © 1966, The Geological Society of America, Inc.

Geological Society of America Bulletin↗

USGS1957

Lithofacies of the salt wash member of the Morrison Formation, Colorado plateau

The Salt Wash is the basal member of the Upper Jurassic Morrison Formation in parts of Utah, Colorado, Arizona, and New Mexico. Deposited by streams, it comprises lenticular beds of cross-laminated sandstone irregularly interbedded with mudstone, siltstone, claystone, and horizontally laminated sandstone. The term "lithofacies," as used in this paper, denotes lithologic aspect. The specific lithofacies of the Salt Wash member at a given locality is determined by the thickness, proportion, and continuity of the stream and flood-plain deposits that make up the Salt Wash. Stream deposits include all rocks interpreted as deposited from moving water; flood-plain deposits include all rocks interpreted as deposited from slack water. Regional differences in lithofacies show that the Salt Wash member is a fan-shaped wedge of sedimentary rocks whose apex is in south-central Utah. Within the wedge, the thickness of the Salt Wash and the thickness, proportion, and continuity of the contained stream deposits decrease relatively uniformly to the north, northeast, and southeast of the apex. Interpretation of the regional differences in lithofacies indicates deposition by a distributary stream system whose apex was in south-central Utah and which spread sediments to the north, east, and southeast over a nearly flat plain. Irregularities on this plain near the Four Corners area and in west-central Colorado modified the distributary system, and therefore the wedge is not symmetrical. Most uranium-vanadium ore deposits in the Salt Wash member occur in a lithofacies near the center of the wedge. This may be a genetic relation and can be explained as a function of transmissibility of the particular lithofacies. The ore deposits, however, are concentrated in a relatively small part of the central lithofacies. Because local geologic features such as structure or igneous intrusions might control the localization of ore deposits in the small area, the high degree of correlation of ore deposits and a certain lithofacies may be coincidental. © 1957, The Geological Society of America, Inc.

Colorado↗

USGS1942

Reconnaissance survey of the Roberts Mountains, Nevada

The Roberts Mountains region, central Nevada, provides an excellent section of Paleozoic rocks ranging from Upper Cambrian to Permian. Major low-angle thrusting is indicated by deformed Ordovician strata resting on Paleozoics of varying age. Overlying a thick breccia zone, the upper thrust plate consists of sandstones, andesitic flows and tuffs, black shales, and bedded cherts (Vinini formation). Ordovician age of the Vinini is established on the basis of graptolite faunules. A belt of Lower to Middle Ordovician graptolitic facies similar to the Vinini formation crosses the Great Basin west of Roberts Mountains. Deposits of roughly the same age in the Roberts Mountains meridian and eastward are dominantly limestone, carrying distinct faunas. Axial planes of overturned folds in the thrust plate dip west, a further indication that the upper thrust plate moved from west to east. Minimum horizontal displacement is 16 miles. The date of thrusting is uncertain, but presumably was later Cretaceous or early Tertiary. Following thrusting, an alaskite stock and rhyolite porphyry plugs were intruded; lava flows and tuffs covered the area in part. Thrust plate and cover of volcanics have been broken into normal fault blocks. The post-thrusting igneous rocks, like volcanic rocks of Utah and New Mexico, are characterized by high potash content.

Nevada↗

USGS1981

Origin and structural implications of upper Miocene rhyolites in Kingston Canyon, Piute County, Utah

Kingston Canyon is one of the deepest antecedent canyons in the High Plateaus subprovince of the Colorado Plateaus. Here the East Fork of the Sevier River flows westward transversely across the gently east tilted Sevier Plateau, which is developed on a basin-range fault block uplifted more than 1,500 m along the Sevier fault zone on the west. Upper Tertiary rhyolites, uncommon in southwestern Utah, occur both on the northern rim and in the bottom of Kingston Canyon. Those on the northern rim consist of lava flows and volcanic domes of the rhyolite of Forshea Mountain, dated by K-Ar methods at 7.6 m.y. old. Those in the bottom of Kingston Canyon, the rhyolite of Phonolite Hill, are especially well exposed and provide spectacular examples of a pyroclastic cone whose base is about at river level and a steep-sided volcanic dome emplaced into and through these deposits. The pyroclastic deposits, formerly 500 or more metres thick, consist of airfall, mudflow, and ash-flow(?) material of rhyolite and foreign lithic fragments, especially olivine basalt. The dome consists of flow-banded, mostly devitrified rhyolite as much as 500 m thick; it has been dated by K-Ar methods at 5.4 m.y. In addition to the rhyolites, a dome and lava-flow complex, the rhyodacite of Dry Lake, occurs near the northern rim and is considered to postdate the rhyolite of Forshea Mountain and predate the rhyolite of Phonolite Hill. The rhyolite of Forshea Mountain was deposited near basin-range faults, before the uplift of the Sevier Plateau and before the cutting of Kingston Canyon. Before uplift, a river flowed across the site of the present Sevier Plateau toward the east-southeast and perhaps also across the Awapa and Aquarius Plateaus to the east. The rhyodacite of Dry Lake was deposited during uplift and perhaps before canyon cutting. During uplift, the river maintained itself and cut Kingston Canyon. The rhyolite of Phonolite Hill was deposited in this canyon, blocking the river flow, which probably formed new outlets to the east. The Awapa and Aquarius Plateaus later were uplifted along faults, disrupting the eastern part of the river segment. The topography then took on its present appearance, and drainage was re-established through Kingston Canyon. There has been little deepening since the reopening of Kingston Canyon.

Utah↗

USGS1971

Metamorphism of Precambrian granitic xenoliths in a mica peridotite at Rose Dome, Woodson County, Kansas: Part 2, petrologic and mineralogic studies

Field, drill-core, petrographic, and mineralogic studies show that the Precambrian granitic rocks exposed on Rose Dome, Woodson County, Kansas, were emplaced as inclusions of basement rock in a mica peridotite magma that intruded the Pennsylvanian section of Rose Dome during Late Cretaceous time. High temperatures of the alkaline ultramafic magma (probably greater than 800° C) led to metamorphism of the granitic inclusions and the formation of high sanidine and high albite from original microcline and albite. Contact metamorphic effects on country rock include the development of buchite-like sanidine-magnesian biotite hornfels from Weston Shale (Pennsylvanian) that was intruded by the mica peridotite. The high temperatures of the peridotite magma also led to partial melting of the granitic rocks; a quartzofeldspathic matrix shows volcanic textures and binds mineral and rock fragments together to produce the varied range of textures and structures of the granitic xenoliths. Partial melting of the granitic rocks accounts for those features that led earlier workers to conclude that the “granite” on Rose Dome had intruded the Pennsylvanian section. Those features included xenoliths of hornfels enclosed by granitic material and apparently intrusive relationships between the granitic rocks and metamorphosed shale. The lack of concordance in the Rb-Sr ages of the granitic rocks (Part 1) may stem partly from melting of the “granite” and exchange of ions with the mica peridotite magma, from the “sampling” of different levels of granitic crust by the upwelling peridotite magma, or from weathering.

Kansas↗

USGS2011

Peninsular terrane basement ages recorded by Paleozoic and Paleoproterozoic zircon in gabbro xenoliths and andesite from Redoubt volcano, Alaska

Historically Sactive Redoubt volcano is an Aleutian arc basalt-to-dacite cone constructed upon the Jurassic–Early Tertiary Alaska–Aleutian Range batholith. The batholith intrudes the Peninsular tectonostratigraphic terrane, which is considered to have developed on oceanic basement and to have accreted to North America, possibly in Late Jurassic time. Xenoliths in Redoubt magmas have been thought to be modern cumulate gabbros and fragments of the batholith. However, new sensitive high-resolution ion microprobe (SHRIMP) U-Pb ages for zircon from gabbro xenoliths from a late Pleistocene pyroclastic deposit are dominated by much older, ca. 310 Ma Pennsylvanian and ca. 1865 Ma Paleoproterozoic grains. Zircon age distributions and trace-element concentrations indicate that the ca. 310 Ma zircons date gabbroic intrusive rocks, and the ca. 1865 Ma zircons also are likely from igneous rocks in or beneath Peninsular terrane basement. The trace-element data imply that four of five Cretaceous–Paleocene zircons, and Pennsylvanian low-U, low-Th zircons in one sample, grew from metamorphic or hydrothermal fluids. Textural evidence of xenocrysts and a dominant population of ca. 1865 Ma zircon in juvenile crystal-rich andesite from the same pyroclastic deposit show that this basement has been assimilated by Redoubt magma. Equilibration temperatures and oxygen fugacities indicated by Fe-Ti–oxide minerals in the gabbros and crystal-rich andesite suggest sources near the margins of the Redoubt magmatic system, most likely in the magma accumulation and storage region currently outlined by seismicity and magma petrology at ∼4–10 km below sea level. Additionally, a partially melted gabbro from the 1990 eruption contains zircon with U-Pb ages between ca. 620 Ma and ca. 1705 Ma, as well as one zircon with a U-Th disequilibrium model age of 0 ka. The zircon ages demonstrate that Pennsylvanian, and probably Paleoproterozoic, igneous rocks exist in, or possibly beneath, Peninsular terrane basement. Discovery of Pennsylvanian gabbro similar in age to Skolai arc plutons 500 km to the northeast indicates that the Peninsular terrane, along with the Wrangellia and Alexander terranes, has been part of the Wrangellia composite terrane since at least Pennsylvanian time. Moreover, the zircon data suggest that a Paleoproterozoic continental fragment may be present in the mid-to-upper crust in southern Alaska.

Alaska↗

USGS2019

Evidence for frequent, large tsunamis spanning locked and creeping parts of the Aleutian megathrust

At the eastern end of the 1957 Andreanof Islands magnitude-8.6 earthquake rupture, Driftwood Bay (Umnak Island) and Stardust Bay (Sedanka Island) lie along presently locked and creeping parts of the Aleutian megathrust, respectively, based on satellite geodesy onshore. Both bays, located 200-km apart, face the Aleutian trench and harbor coastal evidence for tsunami inundation in 1957. Here we describe the evidence at Driftwood Bay, including eight sheets of landward-fining, normally-graded marine sand that extend up to 375-m inland and 23 m above mean tide level. Drift logs that corroborate historical accounts of 1957 tsunami runup on Umnak Island’s Pacific coast overlie the youngest sand sheet, which 137Cs activity shows was deposited in the decade before 1963. The older sand sheets probably record tsunamis prior to 1957 because an emergent coastal terrace lacks evidence for storm-wave erosion and overwash since ~2 ka. Comparisons of the Driftwood Bay and Stardust Bay tsunami histories suggest that at least twice in the past 1700 years inundation occurred at one site but not the other. In contrast, Bayesian age-depth modeling suggests that the two bays may record five tsunamis like the 1957 tsunami, generated by earthquake ruptures that spanned the presently locked and creeping parts of the Aleutian megathrust. However, serial tsunamis occurring within days to centuries cannot be precluded. Our findings imply 164–257-year recurrence intervals for large eastern Aleutian tsunamis and challenge the notion that creeping parts of the megathrust inferred from geodesy onshore pose lower earthquake and tsunami hazards than locked areas.

Geological Society of America Bulletin↗

USGS1972

Pliocene uplift of the grand canyon region - time of drainage adjustment

Tertiary gravel deposits in ancient stream channels along the southern margin of the Colorado Plateaus of northern Arizona show by composition and structure that these deposits came from sources to the south and southwest at a time when central Arizona stood higher than the present Grand Canyon region. Three cobbles of basalt included in the gravel deposits have K-Ar ages of about 10.0 m.y., 12.2, and 12.4 m.y. showing that the major uplift of the plateau in northern Arizona had not taken place at that time. The present south-flowing drainage of the Verde River and neighboring streams resulted from final elevation of the northern Arizona region relative to central Arizona, and must have developed well before about 5 m.y. - the age of some basalts that flowed into the Verde Valley. Thus, the major relative uplift of the southern part of the Colorado Plateaus must have occurred within the 5 to 10 m.y. interval, or in early to middle Pliocene time. This time of uplift also was the time of major canyon erosion, including the cutting of Grand Canyon, within the Plateaus province. © 1972, The Geological Society of America, Inc.

Arizona↗

USGS1967

Stratigraphy and paleoenvironment of the phosphatic miocene strata of North Carolina

Foraminifera and Mollusca collected from the phosphatic Pungo River Formation and the overlying Yorktown Formation in eastern North Carolina were analyzed and interpreted for stratigraphic and environmental significance in order to determine optimum depositional sites for primary phosphorite. The Mollusca and benthonic foraminifera of the Pungo River Formation correlate with those of the Calvert Formation of Maryland, and the planktonic foraminifera in both of these formations correlate with the Globigerinatella insueta zone of Trinidad, postulated as late Aquitanian age. The paleoenvironment of the phosphorite deposition, interpreted primarily from the benthonic foraminifera, was of cool-temperate waters, ranging in depth from 100 to 200 m in the phosphatic beds to less than 70 m in the upper calcareous beds where phosphate is scarce. Phosphorite deposition occurred in an oceanic embayment located south of the Fort Monroe high in southern Virginia and north of a positive feature whose axis lies in the vicinity of New Bern, North Carolina . Cool-temperate waters in this area during Pungo River time indicate that circulation patterns of ocean currents and the resultant faunal provinces were not the same as those at present and later in the Miocene . In the Pungo River and its time equivalents of the Atlantic Coastal Plain, the presence of thick diatomaceous clay units, volcanic ash beds, shards, attapulgite clays, and other minerals probably derived from volcanic rocks, suggests a volcanic source somewhere off the coast during the Miocene . The Yorktown unconformably overlies the Pungo River Formation. The unconformity is marked by channels into the Pungo River, filled with phosphatic pebbles, vertebrate bones, and lower York-town molluscs and microfauna. The coarse-grained phosphatic material is derived from the underlying fine-grained primary phosphorite in the Pungo River and is abundant only in the lower part of the Yorktown Formation. Deposition of the lower part of the Yorktown occurred in waters about 100 m deep. The waters gradually became more shallow as deposition of the formation continued until depths of less than 15 m, and probable brackish conditions, were reached as the uppermost part of the formation was deposited. Temperature of the waters, cool-temperate during lower Yorktown deposition, became warm-temperate to subtropical in later Yorktown time. The faunal patterns suggest that circulation patterns reached their present state during late Yorktown time.

North Carolina↗

USGS1974

Geologic framework of the Kuluncak-Sofular Area, East-Central Turkey, and K-Ar ages of igneous rocks

The Kuluncak-Sofular area, located about midway between Sivas and Malatya in east-central Turkey is underlain by a variety of sedimentary, volcanic, and intrusive rocks. The sedimentary rocks have been deposited on a pre-Campanian serpentinite basement and include Cretaceous conglomerate, graywacke, tuff, and limestone; Eocene arkosic sandstone, conglomerate, and limestone; and Miocene limestone and dolomite. K-Ar ages determined for volcanic and intrusive rocks from the same area are 75.5 m.y. for alkalic diabase that intrudes the Upper Cretaceous sedimentary rocks; 74.3 and 71.1 m.y. for trachyte that partly overlies and partly intrudes the same Upper Cretaceous sequence; 65.2 m.y. for alkalic syenite that intrudes Upper Cretaceous limestone; 18.7 to 16.8 m.y. for andesite and basalt that overlie middle to late Eocene sedimentary rocks; and 14.1 m.y. for a dacite plug that cuts Miocene limestone. © 1974 Geological Society of America.

Geological Society of America Bulletin↗

USGS1971

Aeromagnetic study of the midcontinent gravity high of central United States

A composite map of detailed aeromagnetic surveys over the midcontinent gravity high provides coverage of the 600-mi-long buried belt of mafic rocks of the Keweenawan Series from their outcrop localities in Minnesota and Wisconsin through Iowa and Nebraska. A map of the subsurface extent of the mafic rocks, based on the intricate magnetic patterns, shows that the rocks form a long, semicontinuous block, averaging 40 mi wide and consisting mainly of a sequence of layered flows. This sequence is probably fault-bounded and has been tilted up along the margins, where the linearity of the anomalies indicates steeper dips. The associated clastic rocks, indicated by a smoother magnetic pattern, occur in basins along both sides of the mafic belt and in grabens and a series of axial basins on the upper surface of the block. The well-defined outliers of flows marginal to the main block and the truncation of some of the outermost flow units along a diagonal boundary striking at an angle to them suggest that the present boundaries of the block are postdepositional structural features. The basins and the edges of the block appear to have controlled later, largely vertical movement in the overlying Paleozoic and younger sedimentary cover. Calculated models based on coincident magnetic and detailed gravity profiles along typical cross sections of the midcontinent gravity high show that the block of mafic rocks is steep-sided and as much as several miles thick. The free-air gravity anomaly, which consists of a large positive maximum flanked by minima, averages very close to zero, indicating that this major crustal feature is regionally compensated, although locally each of its components shows a large departure from equilibrium. Remanent magnetization is a primary factor in the interpretation of the magnetic data. Magnetic property studies of Keweenawan mafic rocks in the Lake Superior region show that remanent magnetization may be five times the magnetization induced by the present Earth's field and differs from it radically in direction. This magnetization was acquired before the flows were tilted into their present positions. A computed magnetic profile shows that a trough of flows with such a magnetization and inward-dipping limbs can account for the observed persistent lows along the western edge of the block, the relatively low magnetic values along the axis of the block, and the large positive anomaly along the eastern side of the block. Flows as much as 1 mi thick near the base of the sequence have a remanent magnetization with a nearly opposite polarity. This reverse polarity has been measured on both sides of Lake Superior and is probably also present farther south, particularly in Iowa where the outer units of the block in an area north of Des Moines give rise to a prominent magnetic low. The axis of this long belt of Keweenawan mafic rocks cuts discordantly through the prevailing east-west-trending fabric of the older Precambrian terrane from southern Kansas to Lake Superior. This belt has several major left-lateral offsets, one of which produces a complete hiatus in the vicinity of the 40th parallel where an east-west transcontinental rift or fracture zone has been proposed. The axial basins of clastic rocks are outlined by linear magnetic anomalies and show a concordant relation to the structure of the mafic flows. These basins are oriented at an angle to the main axis, suggesting that the entire feature originated as a major rift composed of a series of short, linear, en echelon segments with offsets similar to the transform faults characterizing the present mid-ocean rift system. This midcontinent rift may well have been part of a Keweenawan global rift system with initial offsets consisting of transform faults along pre-existing fractures, but apparently it never fully developed laterally into an ocean basin, and the upwelling mafic material was localized along a relatively narrow belt.

Geological Society of America Bulletin↗