Charles D. Blome

Phanerozoic stratigraphy of Northwind Ridge, magnetic anomalies in the Canada basin, and the geometry and timing of rifting in the Amerasia basin, Arctic Ocean

Cores from Northwind Ridge, a high-standing continental fragment in the Chukchi borderland of the oceanic Amerasia basin, Arctic Ocean, contain representatives of every Phanerozoic system except the Silurian and Devonian systems. Cambrian and Ordovician shallow-water marine carbonates in Northwind Ridge are similar to basement rocks beneath the Sverdrup basin of the Canadian Arctic Archipelago. Upper Mississippian(?) to Permian shelf carbonate and spicularite and Triassic turbidite and shelf lutite resemble coeval strata in the Sverdrup basin and the western Arctic Alaska basin (Hanna trough). These resemblances indicate that Triassic and older strata in southern Northwind Ridge were attached to both Arctic Canada and Arctic Alaska prior to the rifting that created the Amerasia basin. Late Jurassic marine lutite in Northwind Ridge was structurally isolated from coeval strata in the Sverdrup and Arctic Alaska basins by rift shoulders and grabens, and is interpreted to be a riftogenic deposit. This lutite may be the oldest deposit in the Canada basin. A cap of late Cenomanian or Turonian rhyodacite air-fall ash that lacks terrigenous material shows that Northwind Ridge was structurally isolated from the adjacent continental margins by earliest Late Cretaceous time. Closing Amerasia basin by conjoining sea-floor magnetic anomalies beneath the Canada basin or by uniting the pre-Jurassic strata of Northwind Ridge with kindred sections in the Sverdrup basin and Hanna trough yield similar tectonic reconstructions. Together with the orientation and age of rift-margin structures, these data suggest that (1) prior to opening of the Amerasia basin, both northern Alaska and the continental ridges of the Chukchi borderland were part of North America, (2) the extension that created the Amerasia basin formed rift-margin grabens beginning in Early Jurassic time and new oceanic crust probably beginning in Late Jurassic or early Neocomian time. Reconstruction of the Amerasia basin on the basis of the stratigraphy of Northwind Ridge and sea-floor magnetic anomalies in the Canada basin accounts in a general way for the major crustal elements of the Amerasia basin, including the highstanding ridges of the Chukchi borderland, and supports S. W. Carey9s hypothesis that the Amerasia basin is the product of anticlockwise rotational rifting of Arctic Alaska from North America.

Equivalent radiolarian ages from ophiolitic terranes of Cyprus and Oman

Radiolarian biostratigraphy shows that umberiferous strata overlying the Troodos ophiolite in Cyprus are Turonian in age and are thus essentially contemporaneous with similar strata that overlie the Samail ophiolite in Oman. However, this radiolarian age is markedly older than Campanian isotopic ages measured on the underlying rocks of the Troodos ophiolite. The revised age for the umbers indicates that the Troodos lavas were formed no later than Turonian time. The presence of overlying autochthonous Maastrichtian chalks restricts the emplacement of the Troodos ophiolite to the Late Cretaceous (Santonian to early Maastrichtian).

Limestone and chert in tectonic blocks from the Esk Head subterrane, South Island, New Zealand

The Esk Head subterrane is a continuous belt, generally 10-20 km wide, of tectonic melange and broken formation on the South Island of New Zealand. This subterrane separates older and younger parts of the Torlesse terrane which is an extensive accretionary prism composed mostly of quartzo-feldspathic, submarine-fan deposits ranging from Permian to Early Cretaceous in age. The Torlesse is the most Pacific-ward of several Permian and Mesozoic accreted terranes in New Zealand that record tectonic amalgamation and ultimate accretion against the Pacific-facing Gondwana margin. The Esk Head subterrane of the Torlesse is especially informative because it includes within it conspicuous tectonic blocks of submarine basalt and a variety of basalt-associated seamount and sea-floor limestones and cherty rocks thought to be representative of the subducted plate. Limestones in tectonic blocks are of Late Triassic and probably Jurassic ages and include (1) submarine-cemented, pelagic-bivalve, geopetal packstone-grainstone; (2) brachiopod-bryozoan encrinite; and (3) radiolarian, pelagic lime mudstone. Most of the Triassic blocks have been dated using conodonts which have remarkably low color alteration index (CAI) values ( Paleogeographic inferences drawn from megafossils, bioclasts, and radiolarians, as well as from carbonate cements, indicate deposition of the oceanic sedimentary rocks at paleolatitudes somewhat lower than that of the New Zealand part of the Gondwana margin, but higher than paleoequatorial latitudes. These oceanic sediments and their basaltic substrates were evidently emplaced in the Torlesse accretionary prism following off-scraping from an extensive subducting oceanic plate, probably the Phoenix plate, which was obliquely convergent with the northwest-trending Gondwana margin during Late Jurassic and/or Early Cretaceous time.

Reinterpretation of lower Mesozoic rocks on the Chilkat Peninsula, Alaska, as a displaced fragment of Wrangellia

The southern Chilkat Peninsula is underlain by low-grade metamorphic rocks consisting of a thick unit of greenstone, in part of Carnian age, that is locally overlain by an attenuated section of calcareous sedimentary rocks and chart of latest Carnian to late Norian age and a thick flysch unit of Jurassic and/or Cretaceous age. The Triassic part of this sequence has previously been inferred to be part of the Taku terrane. However, its lithology, age, fossils, and regional distribution indicate that it is probably a sliver of Wrangellia that was offset from the eastern Alaska Range to its present position by at least 350 km dextral displacement on the Denali fault.

Implications of new Jurassic stratigraphic, geochronometric, and paleolatitudinal data from the western Klamath terrane (Smith River and Rogue Valley subterranes)

Combined biostratigraphic, chronostratigraphic, and geochronometric studies of the Rogue and Galice Formations (Rogue Valley subterrane, southwestern Oregon) indicate that the Oxfordian-Kimmeridgian boundary should be placed at 154 ±1.5 Ma rather than at 156 ±6 Ma as advocated in the Decade of North American Geology 1983 geologic time scale. In the Smith River subterrane (northwestern California, southwestern Oregon), well-preserved Radiolaria were recovered not only from strata overlying the Josephine ophiolite, but also from within the volcanic member of the ophiolite. U/Pb geochronometric data constrain the age of the Mirifusus first-occurrence event to after 162 ±1 Ma and to being 157 ±1.5 Ma or slightly older. Radiolarian faunal data indicate that the Josephine ophiolite originated at Central Tethyan paleolatitudes during the latest Callovian (162 Ma) and was carried northward to Northern Tethyan and Southern Boreal paleolatitudes during the Oxfordian.

Implications of upper Mesozoic conglomerate for suspect terrane in western California and adjacent areas

Several distinctive kinds of conglomerate occur in upper Mesozoic rocks in different regions of western California. Within each region, conglomerate in the accretionary-wedge Franciscan assemblage is similar to conglomerate exposed nearby in the fore-arc-basin Great Valley sequence, suggesting that both rock units were derived from the same source regions. Long-distance tectonic transport is not necessary to account for the detrital sedimentary rocks in the Franciscan. The types of conglomerate in the lower part of the Great Valley sequence relate to their place of deposition in the fore-arc basin. In the northern Coast Ranges, chert-rich conglomerate derived from the Klamath Mountains was transported longitudinally southward by a relatively large distribution system. Correlative conglomerate in the Diablo Range is rich in volcanic and detrital- sedimentary-rock clasts derived from the western edge of the Sierra Nevada and transported westward by relatively small distribution systems. Conglomerate rich in chert and quartzite in suspect terrane of the Golden Gate-Gilroy and Nacimiento blocks of central California may have been deposited by a large north-flowing system in the southern part of the same basin; an exotic origin is not required for the suspect terrane. In mid-Cretaceous time, the Nacimiento block may have been displaced southeastward along the Nacimiento fault and juxtaposed against the Salinian block, a deep and interior part of the magmatic arc in southern California. This displacement may be reflected in the clast composition of mid-Cretaceous conglomerate of the Nacimiento block, but stratigraphic documentation is still incomplete. Younger and approximately equal right-slip on the San Andreas fault system brought both the Nacimiento and Salinian blocks northwest to their present position. A southward increase in quartzite content in conglomerate of post-Nevadan age is broadly parallel to a similar increase in pre-Nevadan conglomerate of adjacent parts of the Klamath Mountains, Sierra Nevada, and Peninsular Ranges, making it unlikely that any of these rocks were tectonically derived from remote regions. Radiolarians from chert pebbles in Franciscan conglomerate are similar in age distribution and faunal composition to those from pebbles in the Great Valley sequence, from pebbles in pre-Nevadan conglomerate, and from bedded chert in presently adjacent source areas.

Acid processing of pre-Tertiary radiolarian cherts and its impact on faunal content and biozonal correlation

The numbers of radiolarians visible in thin sections of chert-rich rocks are commonly an order of magnitude greater than the numbers observed on the surfaces of fragments etched by hydrofluoric acid (HF) and typically orders of magnitude greater than the numbers of individuals found in HF-processed residues. Destruction of radiolarians during both diagenesis and HF processing severely reduces faunal abundance and diversity and affects the taxonomic and biostratigraphic utility of chert residues. The robust forms that survive the processing represent only a small fraction of the death assemblage, and delicate skeletal structures used for species differentiation, commonly preserved in limestone radiolarian faunas, are either poorly preserved or dissolved in many coeval chert residues. First and last occurrences of taxa in chert sequences are likely to be coarse approximations of their true stratigraphic ranges. Precise correlation is difficult between biozonations based solely on index species from cherts and those constructed from limestone faunas. Careful selection of samples in sequence, use of weaker HF solutions, and study of both chert and limestone faunas should yield better biostratigraphic information.

Far-travelled Permian chert of the North Fork terrane, Klamath Mountains, California

Permian chert in the North Fork terrane and correlative rocks of the Klamath Mountains province has a remanent magnetization that is prefolding and presumably primary. Paleomagnetic results indicate that the chert formed at a paleolatitude of 8.6° ± 2.5° but in which hemisphere remains uncertain. This finding requires that these rocks have undergone at least 8.6° ± 4.4° of northward transport relative to Permian North America since their deposition. Paleontological evidence suggests that the Permian limestone of the Eastern Klamath terrane originated thousands of kilometers distant from North America. The limestone of the North Fork terrane may have formed at a similar or even greater distance as suggested by its faunal affinity to the Eastern Klamath terrane and more westerly position. Available evidence indicates that convergence of the North Fork and composite Central Metamorphic-Eastern Klamath terranes occurred during Triassic or Early Jurassic time and that their joining together was a Middle Jurassic event. Primary and secondary magnetizations indicate that the new composite terrane containing these and other rocks of the Western Paleozoic and Triassic belt behaved as a single rigid block that has been latitudinally concordant with the North American craton since Middle Jurassic time.

Evolution of a Permo-Triassic sedimentary melange, Grindstone terrane, east-central Oregon

The Grindstone terrane in east-central Oregon is one of the few areas in western North America where large blocks of unmetamorphosed Devonian, Mississippian, and Permian limestones are inter mixed with Permian and Lower Triassic radiolarian chert and Pennsylvanian?, Permian, and Triassic volcaniclastic rocks. Although originally described as parts of a coherent succession, we interpret the Grindstone rocks to be a sedimentary melange composed of Paleozoic limestone slide and slump blocks that became detached from a carbonate shelf fringing a volcanic knoll or edifice in Late Permian to Middle Triassic time and were intermixed with Permian and Triassic slope to basinal clastic and volcaniclastic rocks in a forearc basin setting. Paleogeographic affinities of the Grindstone limestone faunas and volcaniclastic debris in the limestone and clastic rocks all indicate deposition in proximity to an island-arc system near the North American craton. The Grindstone terrane deposits are unconformably overlain by Upper Triassic to Middle Jurassic sequences of the Izee terrane. Although lithologic and faunal differences indicate that the Grindstone and Izee terranes together represent a tectonic block separate from the adjacent Baker terrane, all three terranes were juxtaposed by Late Triassic or Early Jurassic time. We recommend reduction to informal status for the Coffee Creek (Mississippian), Spotted Ridge (Pennsylvanian?), and Coyote Butte (Permian) formations because (1) they cannot be mapped or traced beyond limited areas, and (2) these older rocks are chaotically inter-mixed with younger chert and volcaniclastic rocks. New biostratigraphic data indicate that the bulk of the Spotted Ridge volcaniclastic rocks represent a part of the Triassic Vester Formation of the Izee terrane.

Late Cretaceous pelagic sediments, volcanic ash and biotas from near the Louisville Hotspot, Pacific Plate, paleolatitude ∼42°S

Abstract Dredging on the deep inner slope of the Tonga Trench, immediately north of the intersection between the Louisville Ridge hotspot chain and the trench, recovered some Late Cretaceous (Maestrichtian) slightly tuffaceous pelagic sediments. They are inferred to have been scraped off a recently subducted Late Cretaceous guyot of the Louisville chain. In the vicinity of the Louisville hotspot (present location 50°26′S, 139°09′W; Late Cretaceous location ∼42°S, longitude unknown) Late Cretaceous rich diatom, radiolarian, silicoflagellate, foraminiferal and coccolith biotas, accumulated on the flanks of the guyot and are described in this paper. Rich sponge faunas are not described. ?Inoceramus prisms are present. Volcanic ash is of within-plate alkalic character. Isotope ratios in bulk carbonate δ18O − 2.63 to + 0.85, δ13C + 2.98 to 3.83) are normal for Pacific Maestrichtian sediments. The local CCD may have been shallower than the regional CCD, because of high organic productivity. In some samples Late Cretaceous materials have been mixed with Neogene materials. Mixing may have taken place on the flanks of the guyot during transit across the western Pacific, or on the trench slope during or after subduction and offscraping about 0.5 Ma.