Ralph Moberly

Petrography and K-Ar Ages of Dredged Volcanic Rocks from the Western Hawaiian Ridge and the Southern Emperor Seamount Chain

Alkalic basalt dredged from Yuryaku Seamount in the southernmost Emperor Seamount chain and from the western Hawaiian Ridge at Pearl and Hermes Reef and at two unnamed seamounts 160 and 380 km west of Midway is similar to the alkalic basalt that caps the volcanoes in the Hawaiian Islands. Conventional and 40 Ar/ 39 Ar K-Ar analyses give best weighted mean ages of 42.3 ± 1.6 m.y. for Yuryaku Seamount, 27.3 ± 0.4 m.y. and 26.7 ± 0.5 m.y. for the two unnamed seamounts, and 20.1 ± 0.5 m.y. for the volcano that forms Pearl and Hermes Reef. The data show that the age of the Hawaiian-Emperor bend is about 41 to 43 m.y. Although the volcanoes in the Hawaiian-Emperor chain generally increase in age to the north and west of the island of Hawaii, the measured age-distance relations along the chain are not linear in detail. A phonolite, possibly a differentiated member of a posterosional nephelinic suite and the first found on the Hawaiian Ridge, was recovered from Pearl and Hermes Reef. Samples of analcime tephrite recovered from the unnamed seamount 380 km west of Midway may also be derived from a posterosional nephelinic suite.

Morphology of a large meandering submarine canyon system on the Peru-Chile forearc

Abstract A SeaMARC II side-scan sonar, single-channel seismic reflection, and bottom sampling survey across the Peru-Chile forearc mapped a large submarine canyon system offshore of southern Peru and northern Chile. The main branch of this canyon system extends over 160 km from the shelf break off southern Peru to its termination in the trench off northern Chile. The canyon is 350 to 1100 m wide, and has relief of 150 to 250 m through most of this distance. Within the East Arequipa Basin, the canyon is intricately meandering and has an overall sinuosity of 1.95, comparable to highly meandering subaerial rivers. Meandering is apparently controlled by the basin slope and the characteristics of the turbidity currents that form the canyons. The upper threshold of basin slope for meandering in this canyon is about 15 m/km, which is approximately 10 times greater than that observed for subaerial rivers of comparable size. Bordering terraces at several levels above the canyon floor are remnants of meander belts that developed during early stages of canyon formation. Several cutoff meander loops are preserved on these arcuate terraces. Other sedimentary structures observed along the canyons include levees, overbank deposits, and crevasse splays. The canyon exits the East Arequipa Basin by breaching the forearc structural high along a fault. Downslope of the structural high, the canyon makes a sudden, nearly right-angle bend to the west and continues across the lower forearc slope to the trench axis. This lower stretch of canyon may have formed by headward erosion that pirated the upslope portion of the southward flowing canyon. The lower slope canyon, south of the sudden bend in the East Arequipa Basin canyon, may be a remnant of the lower portion of this ancestral canyon.

MORRISON, CLOVERLY, AND SYKES MOUNTAIN FORMATIONS, NORTHERN BIGHORN BASIN, WYOMING AND MONTANA

The Morrison, Cloverly, and Sykes Mountain formations are the uppermost Jurassic and lowermost Cretaceous sedimentary rocks in the northern Bighorn Basin, Wyoming and Montana. Similar formations were deposited contemporaneously throughout most of the Western Interior. Most studies of nonmarine rocks have been in localities of more rapid sedimentation, whereas the rocks of this study accumulated slowly, under virtually atectonic conditions. The Morrison formation of the Bighorn Basin, as here restricted, includes the earliest-formed nonmarine sedimentary rocks of this sequence, conformably overlying the marine Sundance formation. The rocks are interlensed calcareous quartz sandstones, green mudstones and shales, and subordinate limestones, with locally conspicuous red-banded mudstones. Overlying the Morrison formation, generally with conformity, is the Cloverly formation, as redefined in this report. The lowest of its three members, here named the Little Sheep mudstone members, consists chiefly of bentonitic (montmorillonitic) mudstones in variegated shades of neutral gray, purple, olive, and dusky to pale red. Other typical lithologies include bentonites, cherts, coaly beds, calcareous nodules, and chert-pebble conglomeratic sandstones. The Pryor conglomerate member in the northernmost Bighorn Basin is characterized by black-chert pebbles and rests unconformably on the Morrison formation. Its beds are the stratigraphic equivalent of the mid-Little Sheep conglomeratic sandstones farther south. The upper member of the Cloverly formation, here named the Himes member, comprises three principal lithologies. Commonly at its base is olive-gray and reddish-brown clay-matrixed salt-and-pepper sandstone. Most of the member is variegated reddish- and yellowish-brown and gray kaolinitic claystone and mudstone, containing veinlets and hardpans of iron oxides. Clean quartz sandstones which filled fluvial channels are laced through the claystones. Disconformably overlying the Himes member are sandstones and thinly interbedded, rusty-brown–weathering siltstones, dark shales, and ironstones, here named the Sykes Mountain formation. The Sykes Mountain formation grades into the overlying marine Thermopolis shale. Distinction of stratigraphic units on a lithogenetic basis is believed to eliminate the confusion which existed in previous nomenclature of this sequence. Characteristic primary and secondary structures, clay, accessory, and authigenic minerals, and gross stratigraphic distribution aided the interpretation of the origin and history of these deposits. The Morrison formation accumulated in fluvial, lacustrine, and flood-plain environments from detritus derived chiefly from erosion of sedimentary rocks west of the present Bighorn Basin. The Little Sheep mudstone member and most of the Himes member of the Cloverly formation probably were formed authigenically in seasonal lakes and swamps from weathering of volcanic debris, with their different lithologies due to different drainage conditions and parent ash. The Pryor conglomerate member of the Cloverly formation and lenses of similar conglomeratic sandstones in the Little Sheep member were derived from reworked sedimentary rocks west of the depositional area. Channel-filling clean quartz sandstones of the Himes member were derived from an eastern sedimentary, and perhaps metamorphic, terrain. Thinly interbedded sandstones and shales of the Sykes Mountain formation are tidal-flat and other shallow-water beds deposited at the periphery of the transgressing Early Cretaceous sea. Slow deposition of these well-sorted and mature detrital sediments and close adjustment of authigenic minerals to prevailing environments depended largely on the stable tectonic conditions. During Cloverly time, very little aggradation except of volcanic debris took place, so that soils were formed and preserved.

Hawaiian submarine terraces, canyons, and Quaternary history evaluated by seismic-reflection profiling☆

Abstract The numerous submarine and elevated terraces that fringe shorelines of the Hawaiian Islands have been used as classic examples of mid-ocean Quaternary eustatic terraces. Submarine canyons are important geomorphic features of island slopes. Later reef growth often partly masks both the terraces and canyons. Although difficult to match from one side of an island to the other, some of the terraces have been correlated to successions of higher and lower Quaternary sea levels determined elsewhere in the world. Subbottom seismic reflection profiling now permits a new view of the problem, especially as related to the most recent marine history of Oahu. The geophysical work allows a partial deciphering of former terraces, now buried by younger reefs and sand, and at the same time shows that the heads of submarine canyons do connect with subaerial valleys beneath the succession of Quaternary nearshore deposits. However, the work has disclosed so many additional buried terraces as to raise serious doubts whether it will be possible, without improved techniques of dating the deposits themselves, to unravel the history of Quaternary sea-level changes in Hawaii, much less to correlate them with events recorded elsewhere.

Hawaiian hotspot volcanism mainly during geomagnetic normal intervals

According to their airborne-detected magnetic anomalies, volcanoes forming the islands, banks, and seamounts along the Hawaiian-Emperor seamount chain erupted predominantly during intervals of normal polarity of Earth9s magnetic field. For the past 10 m.y. of hotspot activity, when the field was 50% normal and 50% reversed, 19 of 23 volcanic edifices from Hawaii to Necker Island were dominantly normal, one probably was normal, and three were dominantly reversed. Statistical probabilities, consideration of induction in the present normal field, and possibilities of field reversals during rapid edifice-building nevertheless show that eruptions were not random with respect to the magnetic field. Similar conclusions, where data are fewer, are good for the older Hawaiian Ridge (29 of 36 normal) and are fair for the Emperor Seamounts (3 of 3 known normal). The correspondence suggests that hotspot plumes are generated near the core-mantle boundary. Spreading-center and magmatic-arc volcanism, on the other hand, are mainly upper-mantle processes. The Jurassic and Cretaceous long normal invervals, however, were times of such high heat flux that sea-floor spreading as well as hotspot activity increased. Larger hotspots under continental lithosphere during those normal intervals may have initiated continental rifting.

The sea floor north of the eastern Hawaiian Islands

Abstract Important processes determining the marine geology adjacent to the Hawaiian Islands include volcanism, isostatic adjustment, and sedimentation. The crust under the Hawaiian Islands has subsided 2–6 km under the large shield volcanoes, and the adjacent sea floor has been downwarped accordingly. Although the crust and upper mantle are depressed in the Hawaiian Deep, they apparently are sufficiently elastic to be broadly upwarped in the Hawaiian Arch, outside the Deep. The sediment filling the moat near the islands is a mixture of detrital and authigenic silicates and organic carbonates. It has slumped down the submarine slopes and through submarine canyons. The sediment loses its carbonate content on the deep sea floor. Its authigenic illite, kaolinite, and montmorillonite, as well as its detrital and pyroclastic feldspar, pyroxene, magnetite, and grains of volcanic rock are diluted by pelagic quartz, illite, Radiolaria, and lesser components. Sedimentological studies are interpreted within the framework of the regional tectonic and geomorphic setting. The sea floor about 150 km north of the island of Maui has been proposed as the site for the Mohole project. The site is covered with brown clay that mantles a surface of low relief south of the crest of the Hawaiian Arch. A small fracture north of the site and some low seamounts about 40 km distant are the only obvious features nearer than 100 km to the site. The area is in the western part of the Baja California Seamount Province where there is no indication of gross abnormality of structure, topography, sediment, or history. Therefore, the proposed drilling site north of Maui appears to be in an area that represents typical oceanic crust.

Authigenic Marine Phyllosilicates Near Hawaii

Amorphous to weakly crystalline detrital muds, eroded from tropically weathered basalt, apparently are rapidly forming illite and montmorillonite in the marine environment near the Hawaiian Islands. Seaward changes in alumina-silica ratios, nonexchangeable cations, and mineralogy, as well as young K-Ar ages, support the interpretation that a substantial part of the marine clay near the Islands is authigenic. Other investigators have shown that part of the marine chlorite has replaced gibbsite. The evidence is not sufficient to show what amount of the kaolinite also found in the muds, if any, is authigenic. These results support the theory that reactions between ocean water and silicates are a major factor in alkali ion fixation and p H control of the oceans.

Sedimentation, dredging, and spoil disposal in a subtropical estuarine lagoon

Kaneohe Bay, Hawaii, is an estuary used as a harbor for a military installation and for recreation, fishing, and research purposes. Rapid shoaling of the bay had been reported and attributed to increased stream erosion and sedimentation from the newly suburbanized watershed. Comparison of a 1976 bathymetric survey of Kaneohe Bay with that of a 1927 survey indicates an average shoaling of the lagoonal area of 1.0 m. Average shoaling for the north and middle bay at 0.6m/49 years (1.2 cm yr−1 is lower than for the south bay at 1.5m/49 years (3.1 cm yr−1). The total lagoonal fill in the 49-year period is about 1.95× 107 m3, assigned as follows: 64% carbonate detritus from the reefs as well as growth of living coral and unrecorded dredging spill, 9% recorded dredging spoils, and only 27% terrigenous sediment. Seismic reflection profiles distinguish spoil from natural sediment and show that the infilling sediment is trapped between, burying reef structures built during Quaternary lower stands of the sea. There had been little obvious change between 1882 and 1927 surveys. All information suggests that increased shoaling rates since 1927 are due to reported and unreported disposal of dredge spoil, mainly from 1939 to 1945 for ship and seaplane channels in the south bay, and not from increased runoff and urbanization around the south bay.