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USGS1986

Effect of height and orientation (microclimate) on geomorphic degradation rates and processes, late-glacial terrace scarps in central Idaho

Skip Nav Destination RESEARCH ARTICLE | JULY 01, 1986 Effect of height and orientation (microclimate) on geomorphic degradation rates and processes, late-glacial terrace scarps in central Idaho KENNETH L. PIERCE ; STEVEN M. COLMAN Author and Article Information GSA Bulletin (1986) 97 (7): 869–885. https://doi.org/10.1130/0016-7606(1986)97<869:EOHAOM>2.0.CO;2 Article history Standard View 2.0.CO;2" data-doctype="contentPdf" data-article-id="188507" data-mce-href="https://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/97/7/869/3434736/i0016-7606-97-7-869.pdf"> Open the PDF for in another window Cite Share Icon Share Permissions Abstract Terrace scarps can serve as a nearly ideal natural laboratory for the study of the evolution of slopes. This paper examines the effects of scarp size (height) and orientation (microclimate) by keeping constant variables such as age, lithology, and regional climate. If a scarp degrades as a closed system, and downslope movement is directly proportional to surface gradient , the evolution of the scarp is modeled by the diffusion equation. For a group of scarps of same age and known starting angle, the diffusion-equation model predicts the relation between maximum scarp angle (𝛉) and scarp height ( h ). Late Pleistocene terrace scarps now as steep as 33.25°, as well as measured angles of repose for sand and gravel, require a starting angle as steep as 33.5°. For latest Pleistocene Idaho and Utah scarps, as h increases, 𝛉 is gentler (more degraded) than modeled by the diffusion equation with a constant rate coefficient. The degradation-rate coefficient ( c ) increases tenfold with scarp height; it should not change with scarp height if downslope movement is solely determined by surface gradient (to the first power). Soil wash appears to be responsible for this departure from the diffusion-equation model, for transport rate by soil wash is a function of scarp size (height). South-facing scarps are less vegetated and more degraded than north-facing scarps. For scarps 2 m high, the degradation rate ( c *) on S-facing scarps is 2 times that on N-facing scarps; for 10-m scarps, it is 5 times. The observed dependence of the rate coefficient c * on scarp height can be removed by normalizing c * to values for west-facing scarps of the same height. The residual c * values calculated by this method correlate well with differences in incident solar radiation resulting from the different scarp orientations and maximum gradients. This correlation demonstrates the importance of orientation on slope processes and their rates through the differences in freeze-thaw cycles, soil moisture, and vegetative cover. Scarp morphology may be used to estimate age, if one accounts for the effects of climate and for scarp height, orientation, and lithology. For example, using the dated Bonneville shoreline scarps for calibration and comparing only scarps of equal height, we estimate the Drum Mountains fault scarps to be 9,000 yr old. This age is about twice that produced by previous diffusion-equation calculations that have not accounted for the height as we have here, but it is the same as independent geologic estimates of their age.

Idaho↗
USGS1945

Petrography, structures, and petrofabrics of the Pinckneyville quartz diorite, Alabama

The Pinckneyville quartz diorite complex underlies an area in eastern Alabama extending from the Coosa River in northwest Elmore County northeast through Coosa and Tallapoosa counties into Clay County. Dark-gray, coarse-grained quartz diorite gneiss constitutes the major part of the complex, but there are smaller amounts of granodiorite and granite gneiss. Hornblende-biotite gneisses also occur and are considered to represent metamorphosed basic sills and dikes intruded before the quartz diorite complex. Schist septa lie within the complex and are usually parallel to the regional strike. The sequence of intrusion as interpreted from inclusions and petrographic data is (1) quartz diorite, (2) granodiorite, (3) granite. Small but persistent amounts of allanite and epidote are characteristic of the rocks of the complex. The texture is subporphyritic and shows evidence of protoclastic deformation. A northeast-southwest trend of inclusions, foliation, lineation, and contacts dominates. Primary flow structures are shown in the orientation of inclusions and micas. Secondary structures were superposed on the original flow structures shortly after intrusion and prior to complete consolidation. A prominent secondary structure is a lineation in b. Stretching in b is indicated by boudinage structures, the lineation, and small displacements of dikes. The intensity of the planar structures in the gneiss grades from strong to indistinct or lacking from east to west. Structures in schist inclusions indicate that some folding had taken place prior to intrusion. Thin pegmatite, granodiorite, and granite dikes are common. Most of them are essentially parallel to the regional strike. The petrographic boundary between gneiss and schist is always sharp. On a large scale, the gneiss structures are essentially parallel to the contacts, but slight to marked discordant relations occur at many localities. The schist structures usually parallel the contacts. Petrofabric analysis confirms the structural field observations. The mica fabrics are ac -girdles and are most complete where the foliation is weakest. Microscopic s -planes do not always coincide with the observed foliation. The quartz fabrics are also ac -girdles but are not so well developed as the mica girdles. Quartz orientations cannot always be related to known s -planes. It is concluded that the Pinckneyville quartz diorite complex represents a syntectonic intrusion—that is, that intrusion took place while external or regional forces were still active. The shape of the intrusion was controlled largely by the wall-rock structures which had developed earlier. The rocks show both igneous and metamorphic characteristics because the intrusion took place under regional metamorphic conditions.

Alabama↗
USGS1945

Glaciation of Mauna Kea, Hawaii

Wentworth and Powers have described four stages of glaciation on Mauna Kea, Hawaii. The type localities of the deposits have been studied by the writer, and only the latest or Makanaka deposits can be accepted as definitely glacial drift. The deposits of the first and second stages are paroxysmal explosion deposits. Those of the third stage are a fanglomerate that could have been laid down in early Wisconsin time by floods caused by hot lava melting the icecap.

Hawaii↗
USGS1945

Stratigraphy and structure of west-central Vermont

The lithologic units recognizable in the fossiliferous succession along southern Lake Champlain are structurally continuous with and traceable eastward into the “marble belt” of west-central Vermont immediately west of the Green Mountain Front. They are also traceable northward through west-central Vermont into a succession in northwestern Vermont bounded on the east and west by major thrusts, where they pass laterally northeastward into fossiliferous shales, the faunal zones of which are correlated with those along southern Lake Champlain and in the Hudson and Mohawk valleys. The Cambrian strata are traced into west-central from northwestern Vermont whereas the Ordovician correlation is with rocks in the Mohawk-Hudson-Champlain region. Certain of the Upper Cambrian strata can be correlated with established formations of this age in both of the outlying areas. The structural pattern reflects movements dependent upon the original distribution of sedimentary facies. Interbedded Carnbro-Ordovician limestones and dolomites grade westward into foreland sandstones and eastward into geosynclinal shales. Cambrian and early Ordovician sandstone tongues extend far to the east. Later Ordovician strata of the shale facies overlie the calcareous and sandy succession and are locally unconformable on it. In the Taconic Range allochthonous Cambrian strata of shale facies are superposed upon the autochthonous Ordovician beds. The rocks of the klippe were derived from a zone at least 50 miles east of the present westernmost exposures, arriving there by movements confined largely to the shale facies. Flexural folding and thrusting, effects of alternating competency and incompetency of the foreland sequence, affected the latter succession and possibly the Taconic Allochthone. Breaking of competent strata in the flexures initiated the thrusts; thrusting continued by the stripping of competent from incompetent beds. This was accompanied by counterclockwise rotation of the thrust slices around pivotal zones bordering on a foreland massif to the southwest. Thus rock cropping out in the north-south thrust slices originally extended northeast-southwest. The thrusts are concentric to the Adirondack crystallines and are cut by the Adirondack normal faults; probably they were warped during normal faulting and uplift of the Adirondacks.

Vermont↗
USGS1946

Contribution to the petrography of Haleakala Volcano, Hawaii

The volcanic rocks of Haleakala Volcano comprise, from oldest to youngest, the Honomanu, Kula, and Hana volcanic series. The Honomanu volcanic series consists largely of olivine basalt, with less abundant basalt and picrite-basalt containing large phenocrysts of olivine. In the Kula and Hana volcanic series there are some basalts and olivine basalts, but the characteristic rocks are andesites and picrite-basalt containing abundant phenocrysts of augite as well as olivine. New chemical analyses of six specimens of Kula lavas are given. Five are andesites, and one a picrite-basalt. Each of the six rocks is described in detail. A variation diagram for Haleakala rocks illustrates the alkalic character of the series.

Hawaii↗
USGS1946

Quartz crystal in Brazil

The peak production of Brazilian quartz crystal was reached in 1943 when 2411 metric tons were exported. Minas Gerais, Goiaz, and Bahia are the principal producing states. Minor quantities are obtained from Espirito Santo and Northeastern Brazil. Primary veins and pegmatites yield secondary deposits of eluvial, colluvial, and alluvial origin. The veins range in form from tabular or lenticular bodies, pipes, and pockets to composite lodes and bedded masses. The mineralogy is simple; accessory minerals are few and rare. A small proportion of clear crystals, occurring in vein masses of milky quartz, furnishes the commercial product. Filling, rather than replacement, is believed to be the dominant emplacement process. Pegmatites yield a little commercial quartz. Both residual (eluvial) and water-laid (alluvial) quartz deposits are economically important. Crystals range up to 40 metric tons in weight; those of 1 to 5 tons occur in many deposits. Quartz from every locality studied is twinned on both the Brazil and Dauphiné laws. Phantoms are abundant. Field studies reveal little difference between the final quartz from some pegmatites and that from veins. It is inferred that the veins may grade into pegmatites at depth and that both are products of the same mineralogenic epoch. Present evidence does not justify a closer dating for the formation of the veins and pegmatites than the early Paleozoic. There are few large quartz mines in Brazil. The impressive quartz production during the war years came from many small, scattered workings. Mining methods are primitive, and mechanization was feasible in relatively few areas. The ratio of mine-clean crystal to the total quantity of excavated material varies between 1:1000 and 1:100,000, with an over-all average for the best mines of the order of 1:10,000. Cost of mining ranges from $2 to $10 a kilogram. The average export value during 1944 was $12.50 per kilogram. The potential reserves of quartz crystal in Brazil are large, but actual or visual reserves in known districts are small. Future production capacity will be determined by economic factors.

GSA Bulletin↗
USGS1947

Geology of the Borah Peak quadrangle, Idaho

This report is on result of a long program of geologic investigation in south-central Idaho, undertaken as an aid in the development of the mineral resources of the region. This quadrangle was examined because of the exceptional opportunities for the study of stratigraphy and structure afforded by the Lost River Range, which is the highest in Idaho and contains comparatively few intrusive igneous rocks. The Borah Peak quadrangle differs from the region to the west and northwest in that it contains parts of two sharply defined, narrow ranges elongated in a north-westerly direction, instead of broad, irregular mountain masses, in which the trends of most local topographic units are ill-defined. These ranges, the Lost River and the Lemhi, are flanked by intermontane valleys sparsely populated by stock ranchers. The Lemhi and Swauger quartzites, regarded as of Belt (pre-Cambrian) age, are named. The two Cambrian (?) formations of the Bayhorse region are not recognized in this quadrangle. The Ramshorn slate (Lower Ordovician), widespread and thick there, does not appear to be exposed anywhere in the Borah Peak quadrangle with the possible exception of small areas east of the Elkhorn Ranch, where relations are obscure. The higher Paleozoic formations, named in order of decreasing age, are the Kinnikinic quartzite, Saturday Mountain formation, Laketown dolomite, Jefferson dolomite, Grand View dolomite, Three Forks limestone, Milligen formation, Brazer limestone, and Wood River formation. The Three Forks limestone, nowhere much over 250 feet thick, is the only one of these not recognized farther west. It provides substantial aid in the interpretation of upper Paleozoic stratigraphy. Equivalent beds to the west are presumably grouped with the Milligen. The grit that introduced uncertainties into the correlation of the Wood River and Milligen formations in the southeastern part of the Bayhorse quadrangle is absent here, and both formations have close lithologic resemblances to those in the type localities in the Wood River region. Wood River strata (Pennsylvanian) crop out only in the southwest corner of the quadrangle, and the Brazer may have been the last Paleozoic unit to be deposited over most of the area. Certainly it can have had comparatively little cover over it throughout the Mesozoic. The Brazer is regarded as Mississippian, but some of the fossils in it have Pennsylvania affinities. The carbonate rocks in the Three Forks and later formations are dominantly calcareous, while those in earlier units are dolomitic. Carbonaceous matter is present in most of the formations but is abundant only in the Milligen. The Challis volcanics (Oligocene or Miocene) constitute the first stratified unit to be laid down after Paleozoic deposition ceased, although there was a little intrusive activity late in the Mesozoic. The volcanics were once widely distributed on the flanks of the mountains and are still plentiful in the northwestern corner of the quadrangle, near the head of the valley of the Pahsimeroi, and in smaller areas. They are neither so abundant nor so diversified as they are farther west. They are locally succeeded by an alluvial formation, here termed the Donkey fanglomerate, of possible Pliocene age. Abundant Quaternary glacial and alluvial deposits are present in the larger valleys. The Lemhi and Swauger quartzites were broadly folded before Paleozoic sedimentation began. Apparently the later deformation affected them only enough to render the more impure beds somewhat schistose. The Paleozoic rocks have been folded into closely spaced, asymmetric anticlines, locally broken by thrusts at and near their crests. These folds approximately parallel the trends of the present range, whereas those in the old rocks strike more nearly north. Later deformation twisted some of the folds and produced thrusts of lower dip and greater extent. The folded rocks in the Lost River Range have been arched into an anticlinorium. The original, tight folds may have preceded the emplacement of the Idaho batholith farther west, while the anticlinorium and the late twists and low thrusts result from deformation during the long period in which the batholith came to place and adjusted itself. Local intricately contorted beds in the Brazer limestone are interpreted as the effects of flowage in calcareous material subjected to tangential pressure under comparatively light supercumbent load. The quadrangle contains numerous normal faults of diverse trends. Most of those that can be conclusively demonstrated strike transverse to the trends of the ranges, but especially along parts of the southwest front of the Lemhi Range normal faults along the range fronts may have helped locally to guide erosion of the mountains. Some of the faults have displacements of thousands of feet and are thought to have originated in connection with the low-angle thrusts. Most of these, as well as minor breaks of diverse trends, affect also the Challis volcanics, which shows that renewed movement occurred in Tertiary time. The present mountain masses and broad intermontane valleys are in about the positions occupied by similar features in the early Tertiary. Several incomplete erosion cycles since then have greatly modified the topography but have not obliterated the influence of these ancient land forms. The results of early episodes in the development of the topography are much obscured by the rugged forms that result from active Pleistocene glaciation and later vigorous stream erosion, but modified remnants of the post-Challis and Donkey Hills surfaces can be clearly distinguished. Several less widespread remnants mark intermediate steps in the process. Exceptionally abundant and permeable, coarse alluvial and glacial deposits floor intermontane valleys and choke the larger mountain gorges to such an extent as to interfere with normal erosion and make the surface-water supplies even scantier than might be expected from the climate of the region, which is moderately humid in the mountains and semiarid in the valleys. Active erosion in the high mountains and comparatively static conditions on fans at the range borders result in striking contrasts.

Idaho↗
USGS1947

Stratigraphy of the upper Cambrian, Llano Uplift, Texas

The two formations and eight members that constitute the Upper Cambrian in the Llano uplift of central Texas are described or redefined, and their lithic characters in 19 measured sections are graphically summarized. Standard reference to them is thus furnished. The Riley formation comprises the basal Paleozoic strata of the Llano uplift. Its initial sediments were deposited on a submerged pre-Cambrian terrane having a known topographic relief as great as 800 feet. Its thickness normally averages about 680 feet but ranges from probably less than 200 to about 800 feet. At most places it is subequally divisible between the Hickory sandstone member below and the Cap Mountain limestone member above, with the thin but widespread, glauconitic Lion Mountain sandstone member capping and completing the sequence. The Wilberns formation includes five named members between the Riley formation and rocks of the Lower Ordovician Ellenburger group. It normally averages about 580 feet thick and ranges from 540 to 610 feet thick, but in the southeastern corner of the Llano uplift truncation of the upper beds has reduced it to 360 feet. The thin but widespread, nonglauconitic Welge sandstone member introduces the sequence. Above it is the Morgan Creek limestone member, grading to the succeeding argillaceous beds of the Point Peak shale. At the top of the sequence are the San Saba limestone and Pedernales dolomite members. These two are essentially equivalent and gradational facies, with the Pedernales normally overlying the San Saba.

Texas↗
USGS1947

Radioactivity of the rocks of the batholith of southern California

Determination of radioactivity has been made on 43 rocks, carefully selected from the different mapped units of the complex Cretaceous batholith of Southern California; they range from gabbro to granite. The activity of the gabbro averages about 0.3 alphas/mg./hr., that of the tonalites, 0.8, the granodiorite 1.3, and the granites over 2.0. The activity of the average rock of the batholith is about 0.9 alphas/mg./hr. Most of the rocks have been analyzed, and modal compositions are given. The radioactivity and the percentages of K 2 O, SiO 2 and PbO are plotted on a variation diagram. The variation curve for activity nearly parallels that for K 2 O, and the ratio of activity to K 2 O is about 0.5. The radioactivities of some rocks fall far from the variation curve, and degree of activity correlates with K 2 O content. A less-marked correlation between activity SiO 2 and PbO was also noted. The K 2 O, SiO 2 , PbO, and radioactivity concentrate in the residual liquid during crystallization and differentiation. PbO follows K 2 O closely, as it is concentrated in the K 2 O minerals, but radioactivity is very low in the K 2 O minerals and is greatly concentrated in the accessory minerals.

California↗
USGS1947

Chemical analyses and calculated modes of the Oliverian magma series, Mt. Washington Quadrangle, New Hampshire

Complete chemical analyses, including the spectrographic determination of 44 trace elements, have been made of six representative specimens from each of the six map units constituting the Oliverian magma series in the Mt. Washington quadrangle of New Hampshire. Potash is systematically higher than soda. An increase in silica, which ranges from 56 to 75 per cent, is accompanied by the usual variation of the other principal oxides; lime, magnesia, and total iron decrease in the more siliceous rocks, whereas potash and soda increase at first and then decline. Potash is 1.5 to 3 per cent higher than soda throughout the series. With increasing silica content Cr 2 O 3 , NiO, SrO, V 2 O 5 , and BaO decrease, but ZnO increases. Comparisons with other parts of the world, especially northwestern Europe, suggest some systematic differences from New Hampshire, but the data are insufficient to justify any definite conclusions. The calculation of modes from the chemical analyses necessitates a discussion of the methods involved and the possible errors. It is believed that in these rocks the modes can be calculated with relatively small errors. An unexplained excess of alumina in the modes, ranging from 0.0 to 1.0 per cent, may be due to more sericite and clay minerals in the rocks than the content of water suggests.

New Hampsire↗
USGS1947

Flysch and molasse

By definition European geologists consider a sequence of limestones, sandstones, and shales, the beds of which are thin, regular, and alternating, and which are deposited in a geosyncline or foredeep shortly before a major orogeny, as the flysch . The waste products that accumulate as a deposit flanking mountains and built in part of the deformed flysch make up the molasse . In field practice the groups of sediments called flysch and molasse, or facies of them, are formations in the American sense. However, the Europeans would not recognize all groups of beds deposited in a geosyncline just before an orogeny as flysch; the beds must possess the proper lithologic and bedding characteristics. Europeans have tacitly tied the variable of lithology and stratification to the variable of orogeny, with attendant difficulties. Originally only the first was denoted, but later the second was emphasized in definition if not in field use. Americans have considered the terms chiefly in their orogenic sense and thereby have called certain sequences flysch that do not fully meet the requirements of European usage. The writers are of the opinion that little is gained by the use of the words; by their application no new fact is told or discovery made. They simply elaborate a conclusion by way of analogy. On the other hand, a possibility of confusion is introduced by the use of the terms, and in America it is best to avoid them.

GSA Bulletin↗
USGS1948

Petrography of Iwo Jima

The rocks of Iwo Jima are augite and augite-hornblende trachyandesites, distinctly more alkalic than the ordinary andesites of Japanese volcanoes or the average world andesite. They are described, and chemical analyses are listed. Fumarolic gases have altered some rocks to a mixture of opal and clay minerals and have cemented tuff with opal, calcite, zeolite, and iron oxides.

Iwo Jima↗
USGS1948

Age of the Kingsbury conglomerate is Eocene

The Kingsbury conglomerate and immediately overlying gravels on the east side of the Bighorn Mountains in Wyoming interfinger eastward with “Wasatch” strata. The latter contain Eocene vertebrates, species of which have been found in the Kingsbury conglomerate. In addition, the “Wasatch” strata contain an Eocene flora that includes the floating fern, Salvinia preauriculata Berry, which, because it is not found in earlier beds, permits recognition of the Paleocene-Eocene boundary in the Rocky Mountains and Plains region.

Wyoming↗
USGS1948

Mineralogy and petrology of the currant creek magnesite deposits and associated rocks of Nevada

Cryptocrystalline magnesite occurs as relatively small dense, white masses in a Tertiary volcanic tuff, here named the Currant tuff, in White Pine and Nye counties, 29 miles southwest of Ely, Nevada. The deposits are small, but some of the magnesite is of very high quality with almost no iron or aluminum. However, much of the magnesite is mixed with dolomite and calcite, and a serpentinelike mineral occurs in one of the deposits in sufficient abundance to be of possible use in ceramics. In scattered areas the tuff has been altered by solutions rich in magnesium and bicarbonic acid to dolomite, magnesite, magnesium silicate, and calcite. Mineral and chemical characteristics are the same throughout the entire rock assemblage of flows and tuffs, suggesting that all the rocks have been derived from the same parent magma. The tuff formation, which ranges in thickness from a few feet to over 400 feet, occurs between two groups of volcanic flows here referred to as the lower volcanics and the upper volcanics. The lower volcanics is composed mainly of flows of hypersthene dacite, but in the southwestern part of the area flows of a hypersthene andesite of balsaltic habit occur between the dacite and the overlying Currant tuff. The upper volcanics which overlie the Currant tuff consist of porphyritic quartz latite and an overlying massive latite crystal tuff. The Currant tuff containing the magnesite has both unmodified and reworked tuff members. The formation of hydrothermal dolomite, magnesite, and deweylite is discussed in detail, and the physical chemistry of solutions containing calcium, magnesium, and carbon dioxide is applied in an explanation of the origin of magnesium-rich solutions.

Nevada↗
USGS1948

Lava movement at Parícutin Volcano, Mexico

The lava flows of Parícutin Volcano, Mexico, offer an exceptional opportunity to study details of lava movement, because many flows are accessible throughout their length and can be watched from their birth to the cessation of their movement. Petrographically the lavas show no significant difference from one flow to another or in different parts of the same flow. The maximum lava temperature observed was 1070°C.; the maximum estimated rate of flow was 60 meters per minute; the viscosity is probably on the order of 10 5 to 10 6 poises. The flows start in fissure zones cutting older Parícutin lavas near the southwest base of the volcano. Within a short distance of its orifice a flow is covered with cooled blocks which are broken and slightly abraded as they are carried downstream. Differential movement in a flow takes place principally at its edges, where a reddish breccia and grooved and slickensided surfaces form. Like a glacier, a lava flow develops transverse crescentic ridges and lateral moraines. At its front movement is normally effected by the rolling forward of molten lava within, which carries down cooled blocks from the surface and piles them up ahead of the flow. As a flow dies the lava level lowers in the channel, and small side tongues break through the moraines. Some Parícutin lavas move considerable distances beneath older lava. The longest flow, which covered the town of San Juan in 1944, has a surface of rough curved slabs very different from the blocky surface of other flows but similar to the surface structures developed near lava orifices and by movement of viscous lava through fissures.

Michoacan↗
USGS1948

Submarine geology of Bikini atoll

The program of investigations made in connection with the atomic bomb tests in the summer of 1946 at Bikini Atoll provided a rare opportunity for obtaining information on the physiography and sediments of atolls. The outer slopes around Bikini Atoll rise from a depth of about 2500 fathoms in a long gradually steepening curve. Between 0 and 200 fathoms the average slope is 25°. On the windward side, the reef at the top of the slope is bordered by a narrow 15-fathom terrace, but on the leeward side by a steep, locally vertical, cliff extending to 35 fathoms. Contours of the outer slopes reveal that a flat-topped mass having nearly the area of Bikini lagoon but below 700 fathoms extends to the northwest. The lagoon itself is saucer-shaped and has a maximum depth of 34 fathoms. Around the lagoon and bordering the inner side of the reef and islands is a 12-fathom terrace. Coral knolls rise abruptly from the lagoon floor, and some of them reach to within a few fathoms of the surface. The middle of the lagoon is floored chiefly by algal debris, whereas the shallow edges are covered by algal and foraminiferal sand. Coral comprises a relatively small percentage of the bottom materials. The outer slopes consist of algal debris grading outward to fine sand and finally to Globigerina sand.

GSA Bulletin↗
USGS1948

Reefs of Bikini, Marshall Islands

Systematic surveys made at Bikini before and after the atomi bomb tests (Operation Crossroads) in the summer of 1946 afforded an unusual opportunity to examine the reefs and islands of one of the larger atolls of Micronesia. The existing reef appears to be developed on an older surface that extends as a terrace into the lagoon and beyond the seaward margin of the reef. A sketch map shows the distribution of the terraces in relation to the existing reefs. Reef zonation is briefly described, and a classification of reef types based upon the characteristics of the marginal zone is presented. Illustrations are given from Bikini's reefs and those of near-by atolls. A sketch map shows the distribution of the reef types at Bikini relative to prevailing winds and currents. The processes of reef growth and unusual areas wherein the existing reef appears to be undergoing destruction are also described and figured.

Bikini Reefs↗
USGS1948

Geology and ground-water resources of Iwo Jima

Iwo Jima, in the western Pacific Ocean, consists of Motoyama, a broad volcanic cone, at the north, and Mt. Suribachi at the south, with an undulating isthmus between. Motoyama is largely light-gray-buff tuff. A thick andesitic lava flow under Suribachi, exposed in several places, is overlain by a thick deposit of cinder and scoria. The isthmus (called Tidorigahara by the Japanese) is underlain by more than 200 feet of loose black volcanic ash and fine cinder derived from Suribachi. Several small coral reefs are located about 340 and 110 feet above present sea level. Iwo Jima first came into existence, probably early in Pleistocene time, with the building above sea level of the tuff cone of Motoyama. Quite late in the active life of Motoyama, volcanic activity on the southwestern flank resulted in the formation of Suribachi. This activity may have started with the welling up of the andesitic lava which underlies Suribachi. Following the major eruption of Suribachi, relative sea level changed, and the sea stood about 360 feet higher than at present. The broad cone of Motoyama was beveled; the relative sea level then dropped 240 feet, with minor halts to about 120 feet above present level. As the island rose, Suribachi burst forth in its last stage of explosive activity. Wave erosion cut deeply into the andesite flow of Suribachi, and a prominent bench level was formed 120 feet above present sea level. The Japanese on the island were often faced with serious water shortages. Americans drilled wells and obtained moderately large supplies of usable water. The temperature of the water ranges from 105° to 179° F., and the water is somewhat mineralized. The most favorable area for ground-water development is the isthmus.

Iwo Jima↗