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Geology topics

J. I. Tracey

Publications and source records attributed to J. I. Tracey.

12 recordsLinked to original sources
USGS2017

Ages of subsurface stratigraphic intervals in the Quaternary of Enewetak Atoll, Marshall Islands

Drill cores of Enewetak Atoll, Marshall Islands, reveal six stratigraphic intervals, numbered in downward sequence, which represent vertical coral growth during Quaternary interglaciations. Radiocarbon dates indicate that the Holocene sea transgressed the emergent reef platform by about 8000 yr B.P. The reef grew rapidly upward (about 5 to 10 mm/yr) until about 6500 yr B.P. Afterward vertical growth slowed to about 0.5 mm/yr, then lateral development became dominant during the last several thousand years. The second interval is dated at 131,000 ?? 3000 yr B.P. by uranium series. This unit correlates with oxygen-isotope substage 5e and with terrace VIIa of Huon Peninsula, New Guinea, and of Main Reef-2 terrace at Atauro Island. The third interval is not dated because corals were recrystallized and it is tentatively correlated with either oxygen-isotope stages 7 or 9. The age of the fourth interval is estimated at 454,000 ?? 100,000 yr B.P. from measured 234U 238U activity ratios. This unit is correlated with either oxygen-isotope stage 9, 11, or 13. ?? 1985.

Quaternary Research↗
USGS2014

Submarine geology and topography in the Northern Marshalls

The atomic bomb tests at Bikini in 1946 provided an opportunity to study the characteristics of atolls using modern surveying techniques. The work has shown that many of the important features, both above and below sea level, are definitely related to the direction of the prevailing winds, waves, and currents. Beyond the windward (north and east) reefs of Bikini, the steep outer slope is broken in most places by a terrace at ten fathoms. The margin of the windward reef is a Lithothamnion ridge, cut by strong grooves or surge channels; large islands are developed on these reefs. Lee reefs have nearly vertical outer slopes near the surface; their margins are smooth and are adapted to light surf; occasional storms have eroded large slump areas; and islands are small and few. Passes are largely confined to the southern reefs. The lagoons studied are 25 to 35 fathoms deep and are bordered by a ten‐fathom terrace. The floors are covered with living Halimeda and algal debris surrounded by a belt of foraminiferal and coral sand. Steep coral knolls, some rising nearly to sea level, are scattered in the lagoons. Many flat‐topped seamounts are present in the area. The 14 that were well surveyed rise from 2500 fathoms to depths between 470 and 850 fathoms. ©1949. American Geophysical Union. All Rights Reserved.

Eos, Transactions, American Geophysical Union↗
USGS1967

Drilling on Midway Atoll, Hawaii

Two holes drilled through reef sediments into basalt have established a geologic section through the Miocene. Midway was built above the sea by flows that were weathered and partially truncated in pre-Miocene time. After submergence, volcanic clays were reworked and covered by limestones. Overall submergence was interrupted at least twice by emergence. The limestones have been leached, recrystallized, and partially dolomitized.

Hawaii↗
USGS1967

Holocene changes in sea level: Evidence in Micronesia

Investigation of 33 islands, scattered widely across the Caroline and Marshall Island groups in the Central Pacific revealed no emerged reefs in which corals had unquestionably formed in situ , or other direct evidence of postglacial high stands of sea level. Low unconsolidated rock terraces and ridges of reefflat islands, mostly lying between tide levels, were composed of rubble conglomerates; carbon-14 dating of 11 samples from the conglomerates so far may suggest a former slightly higher sea level (nine samples range between 1890 and 3450 and one approaches 4500 years ago). However, recent hurricanes have produced ridges of comparable height and material, and in the same areas relics from World War II have been found cemented in place. Thus these datings do not in themselves necessarily indicate formerly higher sea levels. Rubble tracts are produced by storms under present conditions without any change in datum, and there seems to be no compelling evidence that they were not so developed during various periods in the past.

Science↗
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↗