Radiolarian Mudstone Research Articles

Petrographic characteristics of deep marine turbidite sandstones of the Upper Cretaceous Tanjero Formation, Northwestern Sulaimaniyah, Iraq: implications for provenance and tectonic setting

This study was carried out to determine the sedimentary provenance of Upper Cretaceous turbidites of Tanjero Formation. The sandstone portion of the unit has been examined based on field and laboratory studies. Seven sections were measured and described in detail on the perfectly cropped out part of the unit at the southern limb of the Sulaimaniyah Syncline. The thickness of the measured sections varies from 120 m to 192 m. The measured sections start from the top of the underlying Shiranish Formation to the syncline axis in the Tanjero Formation. For petrographic analysis sixty- nine representative rock samples were collected. Modal analysis and ternary diagrams point out that, the sandstones are calclithite (litharenite), very fine to medium grained in size consisting of chert, siltstone, mudstone, radiolarian chert and radiolarian mudstone fragments, angular to subangular in shape, very poorly to moderately sorted, transported over short distances and represent submature stage. Grain contact types and high contact index (4.7) indicate moderate to tightly packing, moderate compaction. Transported broken neritic fossil shells, moderately rounded glauconite grains, and undefinable fossils in the altered carbonate rock fragments indicate that the tectonic provenance, lithic recycle category, composed of not only the clastics as interpreted in previous studies derived from Lower Cretaceous Qulqula (radiolarian) Formation which represents deep marine, but also it revealed that a sedimentary formation must also exist in the source area, which is the Lower Cretaceous Balambo Formation.

Cambrian-Eocene pre-rift, pulsed rift, passive margin and emplacement processes along the northern margin of the Southern Neotethys: Evidence from the Antalya Complex in the Alanya Window (S Turkey)

Sedimentary rocks in the Alanya Window document pulsed Permian-Triassic rifting in a proximal basin setting, adjacent to the Tauride continental unit (Geyik Dağ). Late Cambrian-Early Ordovician clastic sediments accumulated along the north margin of Gondwana on a variable shallow-marine shelf. Above an unconformity related to rift-shoulder uplift, Late Permian facies document shallow-marine to evaporitic environments during regional tectonic subsidence (first main rift pulse). Above a second unconformity (both extension and sea-level controlled), Early Triassic carbonates and mudrocks accumulated on an unstable, gently subsiding shelf. Mudrocks, sandstones and lithoclastic debris-flows, derived from the underlying succession, accumulated during the Middle Triassic (Anisian-early Ladinian), implying strong tectonic subsidence and flank uplift (second main rift pulse). Radiolarian mudstones accumulated during late Middle Triassic-early Late Triassic in a well-oxidised, organically productive, but relatively quiescent, deep-water basin above the carbonate compensation depth (CCD). Thick (100s m) lithoclastic sandstone turbidites (commonly plant-rich) and localised debris-flows accumulated during the Late Triassic (Carnian), together with detached blocks of underlying lithologies (third main rift pulse), combined with regional uplift. Alkaline basaltic sills were intruded locally. Final continental break-up to create the Southern Neotethys took place regionally during the Late Triassic (Carnian). Latest Triassic-Late Cretaceous deposition records passive margin subsidence. Variable low-grade metamorphism and two-stage tectonic emplacement (southwards(?) then northwards) took place during latest Cretaceous and Eocene, respectively. The tectonic-sedimentary development of the Antalya Complex provides insights into rift/continental break-up processes that differ from the recently well-documented Alpine-North Atlantic region.

The distinctive tectonic and metamorphic history of the Barru Block, South Sulawesi, Indonesia: Petrological, geochemical and geochronological evidence

This study describes the field relationships, petrography and geochemistry of various metamorphic and magmatic rocks from the Barru Block, South Sulawesi, Indonesia, and reports Rb-Sr isochron ages for clastic metasedimentary rocks. The Barru Block is composed of a tectonic stack of various lithological units, including cherts, radiolarian mudstone, breccias, shales, phyllites, mica schist, gneisses, amphibolite and ultramafic rocks. Dacitic rocks intrude the metamorphic and ultramafic sequences. Greenschist- to amphibolite-facies metasedimentary rocks represent detritus from nearby continental blocks that have been redistributed by turbidity currents. Quartzofeldspathic gneisses are derived from andesitic and dacitic protoliths, whereas amphibolites have basaltic precursors. The clastic metasedimentary rocks show enrichment in LILE and HFSE, particularly Nb, but depletion in Sr. The amphibolites display trace element patterns typical of N-MORB rocks, whereas compositional characteristics of gneisses and dacites suggest that they originated in an arc-related environment.Geothermobarometric calculations and mineral assemblages of greenschist facies-rocks suggest metamorphic equilibration at 250–300 °C and a maximum pressure of 5 kbar. Temperature estimates for amphibolites are 640 ± 20 °C at a pressure of 1.3–3 kbar, whereas the temperature estimates for quartzofeldspathic gneisses range from ca. 630–660 °C for peak conditions to 430–520 °C for the retrograde stage. Internal Rb-Sr mineral isochrons for three medium-pressure mica schists indicate metamorphic ages of ca. 70.7 ± 2.4 Ma, 107.3 ± 0.7 Ma and 110 ± 2.4 Ma, respectively. This age range confirms the importance of Early Cretaceous (Albian) metamorphism within the region, but also indicates a previously unknown Late Cretaceous (Maastrichtian) metamorphic or deformational event at ca. 70 Ma.The Barru rocks show a range of tectonic environments as diverse as that of the adjacent Bantimala Block, but overall, the Barru rock association is more felsic in composition. Although the two blocks accreted approximately at the same time (Early Cretaceous), differences in protoliths, geochemistry and metamorphic P–T conditions imply that they were not geographically close together at that time, but were subsequently juxtaposed tectonically.

Leave no mudstone unturned: Geochemical proxies for provenancing mudstone temper sources in South-Western Cyprus

A variety of ceramic fabrics bearing mudstone inclusions (either naturally existing in the clay or added as temper) are attested in several sites in South-Western (SW) Cyprus. Within the Mamonia terrane of SW Cyprus mudstone-bearing lithologies are divided into two main groups. Sediments of the Ayios Photios group (sandstones, siltstones, mudstones, calcarenites, occasional limestones and chert) were deposited in marine conditions and close to the continental slope. In contrast, the contemporaneous Dhiarizos group contains radiolarian mudstones and cherts deposited in deep-sea conditions. Mudstones and cherts from both formations share similar macroscopic characteristics (distinctive red colour, fine texture) and can be confused for one another, especially when examined only as small-scale ceramic inclusions. Being able to differentiate between the different inclusion types and to link them to one of the two formations leads to useful conclusions regarding the provenance of ceramic samples within the Mamonia terrane.In this case study, geological samples from relevant mudstone sources were analysed as reference materials to describe the possible types of mudstone inclusions. Sr and Pb isotopic data were the primary means for distinguishing between the two major formations, as they closely relate to the differences in depositional setting. Based on the established isotopic classification, microtextural analysis and elemental mapping data were also compared to determine if members of the two formations are distinguishable at this level. Finally, as the isotopic approach proved to be the one offering better results, a theoretical outline for utilizing this type of information to determine the provenance of mudstones in ceramic sherds is presented.

Facies analysis and sea-level change at the Guadalupian–Lopingian Global Stratotype (Laibin, South China), and its bearing on the end-Guadalupian mass extinction

Abstract: The Guadalupian–Lopingian boundary stratotype at Penglaitan, and the nearby Tieqiao section, near Laibin, South China, record a series of major environmental changes within the Jiangnan Basin during a Mid-Permian biotic crisis. The sequence-stratigraphic, petrographic and palaeontological record of these sections has been studied and the associated strontium isotopic fluctuations have been assayed. Mass extinction of fusulinid foraminifers is most clearly associated in time with onset of volcanism and a relative sea-level fall that led to the establishment of mid-ramp conditions (Laibin Limestone) in settings that were previously dominated by radiolarian mudstones. The regression also coincides with a low point of 87 Sr/ 86 Sr ratios. The lowstand deposits contain mafic scoriaceous grains that record pyroclastic volcanism probably centred in the Emeishan flood basalt province 800 km to the west of Laibin. Thus, unusually violent eruptions associated with flood basalts in this province may have contributed to the environmental stresses responsible for the extinction event. Subsequent environmental changes included transgression, spread of dysoxic waters, indicated by populations of small pyrite framboids, and a major negative C-isotope excursion. All these phenomena have been previously related to the end-Guadalupian extinction but they in fact post-date the crisis because a post-extinction fauna of foraminifers is encountered at this time.

Eustatic and palaeoenvironmental assessment of the mid-Cretaceous Bathurst Island Group of the Money Shoals Platform, northern Australia

Abstract The Cretaceous System of the Money Shoals Platform consists of the Bathurst Island Group which is extensively developed on the continental borderland of northern Australia. The platform experienced a simple post-Cretaceous tectonic history and in its southern part the Bathurst Island Group is nonconformable on Precambrian basement, an ideal circumstance for evaluating eustacy and palaeoenvironmental succession. The group records a long-term sea-level cycle of late Aptian-mid-Turonian age. Its lowest part represents a transgressive systems tract developed on a pediment surface of very low local and regional relief. Compositionally mature glauconitic sandstone and radiolarian mudstone are characteristic facies, indicative of terrigenous sediment starvation for the shallow transgressive sea which received wind-driven, plankton-bearing currents from the shelf edge 400 km to the north. A district-scale nodular phosphorite horizon and associated highly condensed section capping the transgressive suite represents the culmination of deepening. It is succeeded by a shelfal mudstone succession comprising the bulk of the Bathurst Island Group and interpreted as a highstand systems tract. Clay mineralogy of this and the underlying radiolarian mudstone is uniform and dominated by smectite/mixed layer species of poor crystallinity. The clay assemblage is attributed to a pedigenic origin associated with weathering of moderate intensity on a poorly drained, low relief landscape. The upper part of the group comprises bioturbated glauconitic and quartzose sandstone displaying sedimentary evidence of high-energy depositional conditions consistent with regression. It is viewed as a prograding/aggrading, shelf margin systems tract which accumulated under circumstances of declining accommodation. A disconformity at the top of the group, cut on strata of mid-Turonian age, completed the sedimentary cycle. Calculated sedimentation rates indicate a major change in detrital sediment supply broadly coincident with the passage from transgressive to highstand systems tract and unrelated to eustacy. The change is attributed to mid-Cretaceous landscape rejuvenation. It is also reflected in the late Albian-Cenomanian sedimentary record of the epicontinental Great Artesian Basin where enhanced sediment supply likewise overprinted and obscured the eustatic signature. Mid-Cretaceous landscape rejuvenation reflected in the sedimentary record broadly correlates with evidence of uplift in eastern and southern Australia now recognised from fission track studies. It also correlates with a widely recognised palaeomagnetic overprint attributed to a mid-Cretaceous thermal event affecting the crust of southern and eastern Australia. Although short-term (third order) sea level cycles are not apparent for the Bathurst Island Group, three eustatic episodes related to global patterns can be recognised: late Aptian transgressive onlap, early Albian maximum flooding and late Turonian regression.

Late Jurassic palaeogeography and anaerobic-dysaerobic sedimentation in the northern Antarctic Peninsula region

Late Jurassic anaerobic-dysaerobic mudstones crop out on both the Weddell Sea (back-arc) and Pacific (fore-are) margins of the northern Antarctic Peninsula. The only known occurrence on the Pacific margin of the Antarctic Peninsula is the Anchorage Formation of Livingston Island. This mudstone dominated unit comprises interbedded volcaniclastic sandstones, pyroclastic/epiclastic tuffs and radiolarian mudstones. The volcaniclastic sandstones are interpreted as representing deposition from turbidity currents. The tuffs represent sedimentation by both primary airfall processes and resedimentation by low concentration turbidity currents. The radiolarian mudstones represent suspen­sion sedimentation, and reveal an upward increase in bioturbation with a transition from anaerobic-dysaerobic conditions to dysaerobic-aerobic conditions. These facies and the observed vertical change in oxygenation conditions are similar to those seen in the Nordenskjöld Formation on the Weddell Sea margin of the Antarctic Peninsula. However, biostratigraphical investigations show that the transition from dysaerobic to aerobic conditions occurred during the late Kimmeridgian-early Tithonian in the Anchorage Formation but late Tithonian or early Berriasian in the Nordenskjold Formation. This diachroneity is related to the palaeogeographical development of the Antarctic Peninsula magmatic arc. A wide epicontinental sea and subdued arc relief in the early Kimmeridgian was followed in the Tithonian by arc uplift, increasing oxygenation in the fore-are basin, and the development of a restricted basin in the hack-arc region. In latest Tithonian-earliest Berriasian times a substantial arc had developed which supplied volcaniclastic sediment to the fore-arc basin; only then was the back-arc basin undergoing the transition from dysaerobic to aerobic conditions. Anaerobic conditions initiated by regional upwelling and expansion of the oxygen minimum zone were perpetuated in a silled basin in the back-arc area, formed by the emergent arc.

Stratigraphy and regional significance of the Upper Jurassic-Lower Cretaceous Byers Group, Livingston Island, Antarctica

The Byers Group, exposed on Byers Peninsula, western Livingston Island, Antarctica, comprises a mudstone dominated sequence at least 1 km thick which accumulated in a marginal fore-arc environment. The basal, 105 m thick Anchorage Formation consists of radiolarian mudstones and tuff-rich interbeds of Kimmeridgian-Tithonian age; it correlates with Upper Jurassic organic-rich mudstone units throughout the proto-South Atlantic region. The succeeding 244 m thick Devils Point Formation marks the first major pulse of coarse volcaniclastic material into the basin. It is in turn followed by the extensive President Beaches Formation, comprising several hundred metres of finely laminated mudstones with at least two major sandstone intercalations. Molluscan and dinoflagellate cyst taxa indicate a Berriasian age and comparatively nearshore depositional environment for this unit. An unconformity of late Berriasian or early Valanginian age separates the three lowest formations from the Chester Cone Formation. The fine-grained Sealer Hill Member at the base of the latter is dated as Valanginian, and grades up into several hundred metres of pebbly sandstones and pebble-granule conglomerates. These mark the second major volcaniclastic pulse and may be of Hauterivian or even younger age. Definition of this major new group will facilitate more precise Upper Jurassic-Lower Cretaceous stratigraphical correlations within the southern South America-Scotia arc-Antarctic Peninsula region. It will also aid our understanding of the critical palaeogeographical transition in the northern Antarctic Peninsula from anoxic basin to active magmatic arc.

The Tethyan Pacific during Late Jurassic and Cretaceous times

During Mesozoic times, the Pacific Ocean occupied almost an entire hemisphere, plus a great wedge of ocean that lay between eastern Gondwana and Laurasia and extended westward from the Pacific into the gradually widening Tethys sea in the Mediterranea, Atlantic, and Gulf of Mexico. As the Atlantic widened, the Pacific narrowed apace, and the Pacific Plate expanded at the expense of bordering plates. During the Late Jurassic and Cretaceous, connections between the Atlantic and Pacific, via passageways between small continental plates in Central America, gradually widened, permitting a globe-circling, mainly west-flowing current system in low latitudes. Except for tectonic slices preserved along the adjacent American and Asian continents, all pre-Jurassic Pacific oceanic sediments have been subducted. Middle and Late Jurassic oceanic crust is inferred from magnetic data, and one drill hole reached Middle Jurassic crust. Pacific Cretaceous pelagic sedimentary rocks are typically nannofossil limestone above the calcite compensation depth and brown, zeolitic clay and radiolarian mudstone below. Chert is common as nodules in both limey and clayey sediments, and siliceous sediments appear to have been deposited over a wide band of latitudes in the Cretaceous. Lower Cretaceous black shale is present on several plateaus, at paleodepths of 500–1500 m, but contemporaneous sediments in adjacent basins are oxidized. Cretaceous vulcanism was on a stupendous scale: in the western equatorial Pacific, a region of about 4x10 6 km 2, centered on the Ontong Java Plateau, is covered almost entirely by mid-plate tholeiitic flood basaltic flows and sills, probably emplaced mainly during the earliest Aptian. Emplacement of these huge masses in a relatively short time may have had important consequences not only for global sea level, but also for the chemistry of the oceans and atmosphere. A striking link to Tethyan facies in the other hemisphere is the presence of shallow-water platform and reefal sediments on the summits of many Pacific Cretaceous seamounts. Several dozen Early Cretaceous seamounts built to heights above sea level, and doubtless many more existed on crust now subducted along the margins of the Pacific. As the seamounts subsided, shallow-water rudist/coral platforms gradually covered them, eventually forming atolls. The platforms are the Pacific expression of the “Urgonian” platforms of the Tethys. Many—perhaps most—of the Pacific Early Cretaceous platforms drowned in the latest Albian, and are guyots today. New volcanic chains rose later in the Cretaceous, but there is a gap in rudist reef development from the Cenomanian through the Santonian. Campanian and Maestrichtian rudist reefs flourished in equatorial latitudes, but hermatypic reefs appear to have been absent during the Paleocene.

Silica Diagenesis in Accreted Eocene Siliceous Rocks (Horizon Ac) on Barbados

ABSTRACT Eocene siliceous rocks exposed on Barbados contain both opal-A (biogenic silcia) and opal-CT, and are interpreted as uplifted abyssal-plain or trench deposits, probably correlative to Horizon Ac beneath the Atlantic Ocean floor. Rocks of hemipelagic origin on Barbados are well layered to massive and consist of radiolarite layers (T layers), matrix-supported radiolarian mudstone (F layers), and layers with an intermediate abundance of radiolarian tests (N layers). Large d-spacings of opal-CT (4.08-4.11 A) and low height to width peak ratios indicate poor crystallinity. Opal-CT is most common in silica-rich T layers, and opal-A is best preserved in mud-rich F layers. D-spacing doesn't correlate with Al2O3 weight percent or layer type as defined by relati e abundance of radiolarians versus mud matrix. We infer that the harder, more indurated T layers had better initial permeability prior to induration and that diagenesis was probably influenced by both porosity and permeability differences and to a lesser extent by chemical and mineralogic differences between F and T layers. Comparable diagenetic states between Horizon Ac siliceous sediments below the Atlantic Ocean floor and accreted sediments on Barbados indicate that accretion of siliceous sediments was not accompanied by deep burial and/or elevated temperatures.

Hemipelagic Rocks at Bissex Hill, Barbados: Sedimentology, Geochemistry, and Depositional Environment

ABSTRACT The island of Barbados exposes the crest of the Barbados accretionary prism of the Lesser Antilles forearc. The sedimentology of Lower Eocene hemipelagite was studied in three fault-bounded packets near Bissex Hill. Three facies were identified based on coherent stratigraphic successions. Facies 1 hemipelagite strata are well layered to massive and consist of radiolarite layers (T layers), matrix-supported radiolarian mudstone (F layers), layers with an intermediate abundance of radiolarian tests (N layers), mud-rich (M) layers containing less than 20% radiolarian tests, rare quartzose sandstone intervals, mud-chip breccia beds, and wispy, laminated, bioturbated units. Many T and N layers have parallel laminations and sharp basal contacts. Few T and N layers have coarse-tail grading, cour structures, and floating mud clasts. Most T and N layers were deposited by turbidity or bottom-water currents. F layers probably represent waning-flow current or suspended-load deposits. Wispy, laminated units are most likely bioturbated T, N, and F layers. Facies 2 strata are wispy, laminated terrigenous mudstones greater than 10 cm thick. Thorough bioturbation indicates slower rates of deposition relative to facies 1 strata. Facies 3 is mudstone composed of sparse radiolarian tests floating in a terrigenous-mud matrix. Facies 3 deposits are interpreted as extensively bioturbated sediment which, prior to reworking, contained fewer intercalated T layers. Bulk-rock chemistry indicates that the hemipelagites were derived from terrigenous and biogenic sources. Weight-percent Al2 UB>O3 correlates with the percentages of clay-sized material in T, N, and F layers. Low total organic carbon (bitumen-free; 0.06-0.83%) and Type III kerogens indicate an oxic depositional environment. The hemipelagic rocks were deposited in abyssal-plain or outer-trench-slope environments.

Radiolarian Biostratigraphy of Hawasina Complex, Northern Oman: ABSTRACT

The allochthonous Hawasina complex is a sedimentary sequence of both continental-slope and oceanic-basin deposits that were thrust up over shallow-water marine carbonates of the Arabian Shelf during the Late Cretaceous. The Hawasina tectonically overlies the autochthonous Hajar Super Group, and is overlain tectonically by the Samail Ophiolite and associated sedimentary rocks. The lower thrust units, the Hamat Duru Group, and Wahrah Al Ayn Formations, are generally interpreted as limestone and sandstone turbidites deposited on the continental rise. The Wahrah Formation and lower Zulla and upper Sid'r Formations within the Hamrat Duru Group all contain thick sequences of radiolarian-bearing chert. Samples collected from the Zulla and Wahrah (lower chert member) Formations yielded radiolarian faunas assignable to the Late Triassic (Karnian/Norian) based on key species of the genera Capnodoce, Capnuchosphaera, Eptingium, Sarla, Triassocampe, and Yeharaia. Additional samples from the Zulla Formation indicate an Early Jurassic (Pliensbachian) age based on the presence of Broctus, Canoptum, Canutus, Droltus, and Pseudoheliodiscus sp.; previous investigators sugge ted a hiatus in pelagic sedimentation during Early Jurassic time. Radiolarian faunas extracted from two measured sections of the Wahrah Formation (upper chert member) range in age from the Late Jurassic (Tithonian) to Early Cretaceous (late Valanginian/Hauterivian). No suitable radiolarian faunas were obtained from cherts of the Sid'r Formation (Hamrat Duru Group). The higher thrust units are represented by the conglomeratic Al Ayn Formation, and the deeper water Halfa and Haliw Formations. Radiolarian faunas extracted from a measured section near the type locality of the Halfa Formation range in age from the Late Jurassic (Kimmeridgian/Tithonian) to the Early Cretaceous (Hauterivian/Barremian). All the radiolarian faunas obtained thus far from the Haliw Formation are assignable to the Late Triassic (late Karnian to middle Norian) based on fragments of Capnodoce and Veghicyclia sp. Previous biostratigraphic data suggested that the thickest sections of radiolarian chert and mudstone were deposited during Late Jurassic and Early Cretaceous time. Newly obtained paleontologic evidence based on radiolarian biostratigraphy indicates that significant pelagic sedimentation occurred also during the Late Triassic and Early Jurassic. End_of_Article - Last_Page 425------------

Early Cretaceous Drowning and Recovery of a Carbonate Continental Margin, Eastern Arabia: ABSTRACT

The eastern margin of the vast Mesozoic carbonate platform which underlies much of Arabia is exposed in the mountains of Oman and the United Arab Emirates. Outer platform and margin facies occur as autochthonous windows in the Oman Mountains and as allochthonous terrane in the Musandam Mountains to the north. These strata are in turn surrounded by westward-thrust nappes of contemporaneous deep-water strata and oceanic lithosphere. Inundation or drowning of the platform occurred in earliest Cretaceous time and is reflected by Jurassic reef limestones overlain directly by Lower Cretaceous pelagic carbonates. In Oman the outer platform strata are buried by over 250 m (820 ft) of these radiolarian lime mudstones. The abrupt upward transition in middle Cretaceous time to ooid grainstones and then muddy fossiliferous limestones interbedded with rudist biostromes documents buildup to sea level and eastward progradation of first high-energy facies and then lower energy interior facies. In the Musandam Mountains to the north, however, where the Jurassic margin proper was drowned, deep submergence was accompanied by flexure and the Lower Cretaceous limestones are slope deposits as indicated by numerous turbidites, debris flows, and slope unconformities. These strata are also transitional upward into shallow water facies so that by Middle Cretaceous time the high-energy facies were once more at the shelf edge. These events, which are reflected along the length of this extensive continental margin, occurred during a time of apparant eustatic sea level rise. End_of_Article - Last_Page 549------------

Miocene Carbonate Gravity Flows in Blake-Bahama Basin: ABSTRACT

Intraclastic chalk, radiolarian mudstone, and carbonate silt (inferred sediment gravity-flow deposits) comprise the Miocene section in the Blake-Bahama basin at DSDP Site 391. Greenish-gray radiolarian mudstones, comparable to hemipelagic sediments that form the North American continental-rise prism elsewhere in the North Atlantic, occur as numerous clasts and a few thin intervals indicating that they formed the background sedimentation. The intraclastic chalk and carbonate silt generally lack sedimentary structures, but gravity-flow deposition is indicated by reworked fossils (forams, End_Page 445------------------------------ nannofossils, and radiolarians) as old as Cretaceous, mixed planktonic and shallow benthic forams, a few shallow-shelf limestone clasts, and preservation of carbonate materials deposited approximately 1 km below the Miocene calcite compensation depth. The Great Abaco Member at Site 391 is 500 m thick. Intermittent coring, recovering 21% of the interval, indicates the following sequence, from top to bottom: (1) structureless white calcareous silt (57 m) with reworked fossils, including Eocene ones; (2) intraclastic marly chalk (123 m) with gray mud clasts of variable abundance and size, up to 2 cm long, attributed to debris-flow deposition; (3) dark, structureless radiolarian mudstone (28 m); probably background hemipelagic sediment, but contains a few reworked fossils; (4) intraclastic chalk (171 m) with a few intervals of dark mudstone and of shallow-water limestone lithoclasts showing lamination, scoured surfaces, and vague grading; debris-flow deposits; (5) heterogeneous interval (124 m) of intraclastic chalk and dark mudstone w th several graded beds of claystone intraclasts which show partial Bouma sequences, suggesting turbidity-current, as well as debris-flow and contour-current deposition. The variety in types of clasts and in age and depth of reworked fossils, as well as the sheer volume of the unit, indicate several source areas including the continental rise and slope (hemipelagic clasts), the Blake Plateau (pelagic carbonate material) and the Great Bahama Banks (shallow-water limestones and benthic fossils). Seismic profiling shows that the Great Abaco Member is about 500 m thick throughout the Blake-Bahama basin, an area of about 50,000 sq km, but pinches out abruptly at the basin margins, probably against hemipelagic muds. Several internal reflectors and a profound regional unconformity at the base (horizon Au, Late Cretaceous to Miocene hiatus) provide good seismic mappability of the Great Abaco Member and several subunits. The internal acoustic character is a series of smooth, closely spaced reflectors, consistent with intermittent gravity-flow deposition, but local current sculpture is indicated by channels and long-wavelength bedforms. End_of_Article - Last_Page 446------------

Cretaceous and Tertiary sediments from the Northwest African Slope (dredges and cores supplementing DSDP results)

Cretaceous and Tertiary sediments dredged and cored on the continental slpe off Cape Bojador (16 stations between 400 and 2800 water depth) provide stratigraphic and regional data not obtained at nearby DSDP Sites 369 and 397, and give additional information on the poorly documented pre-Quaternary litho- and biofacies of the northwest African continental slope. The structural interpretation of the continental margin is based on geophysical, oil exploration and DSDP information. “Middel” Cretaceous (Albian-Turonian) dark (saproelic) shales with calcareous concretions and belemnites reflect an oxygen-depleted environment, marl- and limestones a more oxygenated slope facies. Major sediment types of the Late Paleogene (Middle Eocene to Oligocene), probably separated from the Late Cretaceous by an unconformity, are calcareous diatom and radiolarian mudstones, weakly silicified radiolarian nannomicrites, well-sorted turbidites, and quartz-, mollusk- and phosphate-rich foraminiferal biomicrites. The abundance of pyrite, fish debris, phosphates, diatoms, radiolarians, and of a low-diversity fauna of opportunistic Buliminacea suggests deposition in an oxygen-depleted upper-slope environment under upwelling conditions. Canyon erosion started during Paleocene/Eocene times. Downslope redeposition (grain-by-grain and by slumping) is significant. During the Neogene (mostly Miocene) the uppermost slope and outer shelf was a starved carbonate platform with phosphatized hardgrounds. It was covered by quartzose and phosphatic dolomites, phosphorites, and local dolomite- and phosphate-cemented eolian-sand turbidites. Middle Miocene diatom mudstones from the intermediate slope reflect upwelling conditions. Major erosion of the lower-slope canyons took place during the Late Oligocene to Early Miocene. It stimulated debris flows which deposited pebbly mudstones with a mixed variety of lithofacies and ages (Albian to Miocene) indicating the extent of erosion into the lower slope.

The Kannaviou Formation, Cyprus: volcaniclastic sedimentation of a probable late Cretaceous volcanic arc

In SW Cyprus the upper pillow lavas of the Troodos Igneous Complex are directly overlain by up to 75° m of volcaniclastic sediments, ranging in age from Lower Campanian to Mid-Maastriehtian. The basal sediments are pre-dominantly non-calcareous bentonitic clays and radiolarian mudstones representative of relatively slow deposition, probably below the carbonate compensation depth. In contrast, stratigraphically higher calcareous bentonitic clays, volcaniclastic siltstones and volcaniclastic sandstones show evidence of much more rapid deposition by a combination of mass flow, traction and minor turbidity currents. The basal volcaniclastic sandstones were partly derived by submarine erosion of the subjacent Troodos lavas. In contrast, the overlying sequences contain abundant fine-grained pumice, basic to acid volcanics, and also terrigenous material derived predominantly from outside the present area of southern Cyprus. The hetero-geneous provenance of the Kannaviou sediments can be explained by postulating late Cretaceous subduction of Troodos-type oceanic-crust with genesis of a related volcanic arc and eventual tectonic juxtaposition of the Troodos Massif with a Mesozoic continental margin.

The origin and diagenesis of cherts from Cyprus

ABSTRACTThe Troodos Massif of Cyprus is overlain by a variety of cherts in pelagic chalks, volcanogenic sediments, radiolarites and radiolarian mudstones, all of Campanian to Upper Eocene age.There are two chert types, granular chert and vitreous chert. X‐ray diffraction (XRD) reveals the silica polymorphs, disordered cristobalite and quartz.Silicification of the chalks varies from incipient, to bedded, granular cherts, all with disordered cristobalite as the main silica phase. Quartzitic cherts are restricted to the base of Upper Palaeocene and Lower Eocene calciturbidite beds. Disordered cristobalite predominates in the radiolarian mudstones at the foot of the sequence.The form of disordered cristobalite in cavities ranges from microspherules of radiating bladed crystals, the ‘lepispheres’ of the Deep Sea Drilling Project (DSDP) to bladed overgrowths, and fibrous silica. In contrast, within the fine grained matrix, the disordered cristobalite takes the form of partly coalescent crude microgranules and microspherules.Most of the chalcedonic quartz in Cyprus is derived by recrystallization of previously inorganically precipitated disordered cristobalite rather than by direct precipitation.According to the concept of impurity‐controlled maturation the composition of host sediment controls the incorporation of exchangeable cations and other impurities into inorganically precipitated disordered cristobalite. With time (up to 100 million years) internal solid state reorganization of the disordered cristobalite is accompanied by gradual expulsion of impurities, until the cristobalite dissolves followed by quartz precipitation. Complete conversion to quartz takes place first in porous calcareous sediments free of impurities, as in the Cyprus calciturbidites; in fine grained clay‐rich sediments, like Cyprus radiolarian mudstones, disordered cristobalite persists much longer. Impurity‐controlled maturation also helps explain the diagenesis of Cyprus chert nodules.

Sedimentation on Cretaceous Ocean Ridge, Troodos Massif, Cyprus: ABSTRACT

The igneous rocks of the Troodos massif have been interpreted as an uplifted segment of Cretaceous ocean crust, formed at a spreading ridge. If so, the sediments overlying them should be similar to those forming at ocean ridges and to Tertiary basal sediments found by JOIDES drilling. Directly above the Troodos pillow lavas are iron- and manganese-rich mudstones (umbers) that are closely comparable to the iron-rich sediments associated with the latest stages of volcanism on present ridges. They are enriched in trace elements, including copper, molybdenum, lead, zinc, and vanadium. These mudstones pass upward into Campanian radiolarites and radiolarian mudstones with a diverse and well-preserved siliceous microfauna. Silica diagenesis can be compared with that of deep-se cherts. In the simpler sequences, the radiolarian rocks are overlain by Maestrichtian chalks. In some areas, illite-montmorillonite clays above the radiolarian mudstones include major developments of melange, with transported blocks of diverse Mesozoic sedimentary and igneous rocks. These include quartz sandstones of continental derivation. Chalks also overlie the melange. The umbers, radiolarites, and chalks are interpreted readily as oceanic sediments, and the presence of the melange implies proximity of a continental margin. End_of_Article - Last_Page 785------------

Review of Stratigraphic Names and Megafaunal Correlation of Pliocene Rocks Along Southeast Margin of Los Angeles Basin, California: ABSTRACT

Three named stratigraphic units that are wholly or in part of Pliocene age form discontinuous outcrops around the southeast margin of the Los Angeles basin: the Fernando Formation (lower and upper Pliocene), the Capistrano Formation (upper Miocene and lower Pliocene), and the Niguel Formation (upper Pliocene). The Fernando Formation is exposed in the Puente Hills, in the Northwestern Santa Ana Mountains, and at Upper Newport Bay; it consists of two stratigraphic units that typically are separated by an unconformity. The thickness of the formation ranges from nearly 6,000 ft in the western Puente Hills to about 1,300 ft at Newport. The common lithologic types in the upper unit are siltstone, sandstone, and conglomerate; the lower unit is predominantly siltstone. Gaps in the faunal record preclude accurate zonation, but much of the provincial Pliocene is represented by a composite section. Using a twofold chronology, the lower unit at most localities is early Pliocene in age, and the upper unit, late Pliocene, although a precise time boundary is difficult to define on the basis of mollusks. M gafaunas from the northeast part of the basin characteristically indicate inner sublittoral environments, whereas those nearer the present coast, which contain large displaced assemblages, suggest deposition in outer sublittoral to upper bathyal depths. Basinward, the Fernando Formation grades into subsurface strata that commonly are called Repetto and Pico Formations. The Capistrano Formation is exposed at Newport Bay and in the Capistrano syncline. Most of the formation at Newport is composed of mudstone, but near Dana Point it consists of a radiolarian mudstone facies; a deep-sea, fan-valley, coarse-grained facies; and a foraminiferal mudstone facies. In the vicinity of San Juan Capistrano, the type area, the formation is about 2,100 ft thick and is composed chiefly of mudstone. Between El Toro and Arroyo Trabuco, the Oso Member, a large lenticular sandstone unit as much as 1,400 ft thick, constitutes most of the formation. The lower part of the Capistrano Formation and the Oso Member are assigned a late Miocene age; the upper part in the San Juan Capistrano-Dana Point area is early Pliocene and locally separated from the lower part by an unconfo mity. At Newport Bay the upper part is eroded, and the formation is entirely late Miocene in age. Small assemblages of mollusks from the upper part of the formation near Dana Point are displaced, and their species content suggests deposition in the upper bathyal zone. The lower part (Miocene) of the Capistrano Formation probably correlates with the Malaga Mudstone Member of the Monterey Shale and with the upper part of the Puente Formation; the upper part (Pliocene), with the lower unit of the Fernando Formation at Newport Bay and in the Puente Hills. The Niguel Formation caps most of the low hills between El Toro and San Juan Capistrano. Much of the formation at the northernmost and westernmost outcrops may be nonmarine, and the uppermost beds in the type area 5 mi north of San Juan Capistrano may be nonmarine. The chief constituents of the formation are sandstone, conglomerate, and siltstone. Conglomeratic beds at the base are deeply channeled into older rocks. A thickness of 350 ft is estimated for the formation in the type area. Mollusk assemblages contain many species diagnostic of a Pliocene age, and the composite fauna is provisionally assigned to the latter half of the epoch. Mixed depth assemblages are common, but the localities in the inland area contain an abundance of shallow, warm-water species, whereas those nearer th coast contain some extant species that range into upper bathyal depths. The composite megafauna is strikingly similar to those from the San Diego Formation at Pacific Beach and from the upper member of the Fernando Formation in the eastern Puente Hills; it closely resembles those from the upper part of the Pico Formation in the eastern Ventura basin and from the Careaga Sandstone in the Santa Maria district. End_of_Article - Last_Page 438------------

Graptolite-Bearing Ordovician Siliceous and Volcanic Rocks, Northern Independence Range, Nevada

The northern Independence Range of northeastern Nevada has a well-exposed succession of siliceous and volcanic rocks that are rich in Ordovician graptolites. This succession is divided into three new formations that are considered subdivisions of the widespread Valmy Formation, which is raised to group status in this paper. The highest peaks of the northern Independence Range are composed of about 1000 feet of massive white quartzite containing interbedded chert and graptolitic shale, the McAfee Quartzite of medial Ordovician (Llandeilan to Caradocian). A similar thickness of bedded chert, pillow lava, and argillaceous rock, the Snow Canyon Formation, containing older lower Ordovician (Arenigian) graptolites is exposed in deep canyons below the quartzite succession. Locally, the upper part of the Snow Canyon Formation has many interbeds of dark quartzite bearing somewhat younger, probably earliest medial Ordovician (Llanvirnian or Llandeilan) graptolites. The Jacks Peak Formation, overlying the McAfee Quartzite, consists of a lower, predominantly black chert member about 200 feet thick and an upper quartzite member of more than 200 feet. Graptolites from the lower member of the Jacks Peak are late medial Ordovician (upper Caradocian). The Jacks Peak Formation is overlain unconformably along Jack Creek north of the high range by the Carboniferous and Permian? Schoonover Formation (Fagan, 1962) of radiolarian chert, mudstone, conglomerate, and volcanic rocks. To the south, the Seetoya sequence of cherts, argillaceous rocks, and volcanic rocks that is thrust over lower Paleozoic carbonate rocks contains middle Ordovician (Caradocian) as well as the lower Silurian (Llandoverian) graptolites previously reported (Kerr, 1962). In the northern Independence Range, representatives of the standard graptolite zones are recognized, permitting correlation of the new formations of the Valmy Group with the following graptolite-bearing siliceous assemblages in widely separated parts of Nevada and Idaho: undivided Valmy and Vinini Formations and Basco, Petes Summit, Sams Springs, Perkins Canyon, and Phi Kappa Formations. Contrasting with this siliceous assemblage is about 4000 feet of Permian and Carboniferous? calcareous sandstone, siltstone, and limestone that unconformably overlies Cambrian carbonate rocks in a triangular fault block at the northeastern tip of the range. This block seems autochthonous with respect to the surrounding siliceous and volcanic rocks. The placement of the siliceous sequence into the present position apparently was accomplished by post-Carboniferous thrusting, for the late Paleozoic rocks in the two sequences contrast in facies, and each lies unconformably on similarly contrasting early Paleozoic rocks although they are of different systems. A fault zone exposed along the eastern base of the range has blocks of Ordovician, Silurian?, and Carboniferous limestones in fault-bounded wedges within the Snow Canyon Formation allochthon and has autochthonous Carboniferous-Permian limestone on the east. This fault may be the northern continuant of the Seetoya thrust that is exposed farther to the south.