Basaltic Breccia Research Articles

Results from a geological reconnaissance around Svartenhuk Halvø, West Greenland

Svartenhuk Halvø is built up primarily of Tertiary basalts; these overlie Cretaceous and early Tertiary sediments, and overlap onto the Precambrian basement. The basalt series can be divided into a lower and an upper series; although displaced by faults, the boundary between these series can be followed across the peninsula. The thickness of the lower basalt series is estimated to be about 2-3 km in the south of the peninsula and barely 1 km in the north; the sub-aquatic basalt breccia is included in these thicknesses. FauIts causing repetitions of the lava succession have resulted in the series being preserved over a rather large area. The general dip of the lavas is 3-4 dregrees towards SW in the east and 8-10 degrees, also towards SW, in the west. Locally dips between 10 and 20 degrees or even steeper are seen; these are the resulf of drag along fault zones in Arfertuarssuk fjord and Kugssineq valley, and between Svartenhuk Halvø and Ubekendt Ejland. The youngest fault has a displacement of 500 m or more and has downthrown the basement area to the north-east in relation to the sediment-basalt breccia-basalt series to the south-west. The upper basalt series has by far the greater lateral extent and covers the gneiss and metasediment area to the north and north-east at least as far as the Inland lee. The dip of the flows in this part of the basalt series is considerably lower than in much of the lower basalt series, but faults repeating the succession are also frequently encountered within the upper basalts. The tectonic movements evidence a strong E-W (or NE-SW) tension, never a compression; the weak anticlinal and synclinal structures which are seen are interpreted as resulfing from differential sagging. The lower basalt series is thought to have arisen from fissure eruptions, with the main area of eruption in the east. The lavas are very rich in olivine (i. e. are pieritic). The upper basalt series probably arose from central eruptions and smaller fissure eruptions, and the area of eruption is thought to have shifted to the west. The upper lavas become poorer in olivine; andesitic lavas represent perhaps a closing phase, more local in its distribution and perhaps resulting from magmatic assimilation of pre-basaltic sediments. "Iron basalt" and intrabasaltic breccia have not been noted on Svartenhuk Halvø.

Oceanic Crust and Mantle Fragment in Subduction Complex near San Luis Obispo, California

A remnant of Mesozoic oceanic crust and mantle overlain by marine sediments occurs northwest of San Luis Obispo, California. The remnant is composed of four principal units. The first (lowermost) unit is serpentinite derived in part from harzburgite, and is more than 1 km thick. The serpentinite, which is separated from the next preserved unit by faults, contains diabase dikes that presumably fed overlying intrusive and extrusive masses. The second unit is largely made up of basaltic breccias and pillow lavas, but the lower part contains countless dikes and sills of diabase, gabbro, trondhjemite, and small amounts of quartz albitite. The collective thickness of the second unit exceeds 1,200 m. The third unit is bedded radiolarian chert 5 to 130 m thick, in depositional contact with the basalt. The chert is overlain concordantly by the fourth unit, consisting of more than 660 m of shale—the Toro Formation of Fairbanks (1904). The lower part of the shale contains latest Jurassic species of Buchia, and the remainder contains Early Cretaceous species of this genus. The oceanic crust and overlying Mesozoic sediments comprise a coherent synclinal remnant which is tectonically underlain by chaotic Franciscan rocks and tectonically overlain by Upper Cretaceous Great Valley-type sandstone. The remnant probably “floated” like a raft while the Franciscan melange passed under it in the upper part of a Subduction zone and dragged it beneath the Upper Cretaceous sandstone.

Nepheline Gabbro from the Mid-Atlantic Ridge

IN the course of a long range programme aimed at the study of the petrology of the Equatorial Mid-Atlantic Ridge, a series of dredgings were made by Miami's Institute of Marine Sciences on the walls of the Romanche trench. The Romanche is a major tectonic fracture which intersects the Ridge close to the Equator, and offsets its axis, perpendicular to its elongation. At one locality on the north wall of the trench (station P6707-25; 00° 22′ S, 20° 09′ W) about 120 kg of rock fragments was recovered from 5,100–5,300 m below sea level. The dredged material consists of about 95 per cent (by weight) of basalt and glassy basaltic breccias; 2.5 per cent of gabbros; 2 per cent of cataclastic breccias; 0.5 per cent of serpentinized peridotite. Among the gabbros norites, troctolites and gabbros with alkali affinities were observed; among the latter, some contain modal nepheline and are similar to theralites. These alkali gabbros are of special interest, particularly because rocks with modal nepheline have not been reported in the past from oceanic ridges. We present here a description of them and a brief discussion of their origin.

Diorites from the Mid-Atlantic Ridge at 45°N

Diorites, associated with basalts, basalt breccias, and serpentinized peridotites, occur in situ on the faulted scarps of two seamounts from the western High Fractured Plateau of the Mid-Atlantic Ridgeat 45 degrees N.

CENOTYPE EFFUSIVES FROM THE TRANS-URAL REGION

Early Mesozoic and possibly Permian extrusive and sedimentary rocks are rather common in the central and southern Trans-Uralian Region. Chemical analyses are given for 3 rhyolites, 19 basalts and andesites, and 3 dolerites. A basalt breccia (167 m.y.) and two rhyolites (164 and 184 m.y.) the geological data of which are well established, have been dated as noted by the K-Ar method. Ranges of certain associated fossils are tabulated. The fossils indicate the dated rocks to be Triassic. R. V. Dietrich

Apercu sur le Quaternaire de Salvador (Amerique centrale)

Abstract El Salvador is essentially a volcanic region in which an older, presumably late Tertiary, complex of andesite and basalt flows and breccias and younger, more acid Quaternary rocks are represented. Following a long period of inactivity during which the Tertiary volcanic masses were considerably eroded, episodic explosive activity occurred in the Quaternary, accompanied by the formation of extensive calderas and ejection of considerable ash. Paleosols were developed in the intervals between explosions, which permit relative dating of the successive episodes. The last stages of activity were characterized by extrusion of mud flows, torrential gullying, and deposition of thick piedmont detrital beds accompanied by reworking of volcanic ash which was redeposited in dammed lakes and other depressions. The development of calcareous crusts in places constitutes evidence of significant climatic fluctuations. There is also evidence of differential subsidence in coastal areas.

Contribution to the stratigraphy of the non-marine Tertiary deposits on the south coast of the Nûgssuaq Peninsula, Northwest Greenland

The Tertiary, sedimentary sequence on the south coast of the Nûgssuaq peninsula, North Greenland (The Upper Atanikerdluk Formation) is described. The type area on the south-eastern part of the peninsula (the Atanikerdluk area) shows the Upper Atanikerdluk formation divided into 5 members: Plateau basalts 5. Point 976 member 4. Aussivik member 3. Umiussat member 2. Naujât member 1. Quikavsak member Cretaecous (Atane formation) The Plateau basalts rest upon the Upper Atanikerdluk formation which again rests unconformably upon the Atane formation (Cretaceous). The first occurrence of subaeric lavas is found in Aussivik member. West of the type area (Pautut and more westerly) the basalt volcanism began earlier, and here the number of sedimentary members of the type area substituted by products of the volcanism (basalt breccia and Plateau basalts) is increasing the more westerly one comes. The basal Quikavsak member is only visible locally owing to its origin as an infilling of a river-valley in connection with the Paleocene transgression. This fossil river-valley can be traced from Atanikerdluk (purely fluviatile) by Pautut to Nûk kitdleq, where a marine intercalation has yielded a minor fossil fauna making the Paleocene age probable by its accordance with an occurrence of marine, fixed Lower Paleocene fauna in a delta deposit at Angmârtussut (the Agatdalen valley), whose stratigraphic equivalence to Quikavsak member is proved. With 1) this fixed point in the Agatdalen valley, 2) the faunal agreement between this occurrence and Nûk kitdleq, 3) the agreement between the fossil flora of the fixed occurrence at Angmârtussut and Quikavsak member at Atanikerdluk and the other occurrences of this member at the south coast of the Nûgssuaq peninsula as well as 4) the same position in the geological course of events, all these occurrences of Quikavsak member and the litho-stratigraphically equivalent occurrences at Angmârtussut are linked together as Lower Paleocene deposits. This dating thus applies to the known fossil floras "Upper Atanikerdluk A" and "Upper Atanikerdluk B" of O. Heer. A short survey of the fossil floras from the localities described is given. Finally, Macclintockia Kanei (Hr.) Sew. & Conw. is stated zone fossil of the Lower Paleocene of West Greenland.

STRATIGRAPHIC SEQUENCE IN THE EAGLE ROCK VOLCANIC AREA NEAR AMERICAN FALLS, IDAHO

Excellent exposures in Snake River canyon in the Eagle Rock area near American Falls, Idaho, were supplemented by numerous drill holes in connection with the exploration for a damsite. The rocks are Lower Pliocene (?) Neeley lake beds and associated basaltic and siliceous sediments and tuffs possibly correlative with the Salt Lake formation to the east and the Payette formation to the west. Above the Neeley formation a welded rhyolite tuff (ignimbrite) contains lithophysae and spherulites and shows four distinct phases. Next higher is a siliceous tuffaceous shallow-water sediment, a thin-bedded sublacustrine basaltic tuff, with very thin diatomaceous interbeds, and finally thick basaltic tuffs, breccias, dikes, and lava flows of the Massacre volcanics. Agglutinated basaltic tuffs are also present. Unconformable on these rocks are the Raft lake beds, possibly correlative with the Lake Idaho deposits farther west, of late Pliocene age. A major erosional unconformity separates the Tertiary rocks from the late Pleistocene Cedar Butte basalt which displaced Snake River for 55 miles and produced a lake 40 miles long and 12 miles wide. The beds deposited by this lake constitute the American Falls formation. Abandoned spring coves cut in the downstream end of the Cedar Butte basalt testify to heavy leakage from this ancient lake. The region was warped into gentle anticlines and synclines near the close of the Tertiary and faulted into small blocks. Several erosional unconformities were found. The petrography is described.

Siletz River Volcanic Series, Northwestern Oregon

The name Siletz River volcanic series is proposed for more than 3,000-5,000 feet of basaltic flows, flow breccia and tuff, in which are included a few beds of clastic sedimentary rocks that contain a fauna correlative with the Capay shale of California. The volcanic series is correlated with the Umpqua formation of southwestern Oregon and with the lower part of the Tillamook volcanic series of northwestern Oregon.