Search

The Nauru Basin contains a well-documented pattern of Late Jurassic magnetic anomalies and a Cretaceous volcanic complex that has not disturbed the older magnetic structure. The following hypotheses are offered to explain this paradox. The Cretaceous basalts are normally and uniformly magnetized, making them invisible to a surface-towed magnetometer. Tension cracks and fractures associated with thermal uplift and/or magma wedging provided pathways for the Cretaceous magma through the Jurassic basement without disrupting the Jurassic magnetic structure. The same tension cracks admitted sea water that convectively removed heat from the magnetic layer and thus maintained the Jurassic basement below its magnetic blocking temperature for extended periods of time during intrusive thermal episodes. Finally, the maximum uplift associated with Cretaceous volcanism in the Nauru Basin could have been equal to, but was probably something less than, that associated with nearby seamount and atoll chains.

Rocks dredged from a seamount 100 km northwest of Caroline Island, at the southern end of the Line Islands chain, contain Late Cretaceous fossils associated with volcanic debris. This association is evidence for the existence of a reef-bearing volcanic edifice with a minimum age of Late Cretaceous, 70 to 75 m.y., near Caroline Island. With the discovery of this seamount, the known occurrences of Late Cretaceous, reef-capped, volcanic edifices now extend a distance of 2,500 km, from Deep Sea Drilling Project Site 165 to 100 km northwest of Caroline Island. The apparent synchroneity of Late Cretaceous volcanism over this distance argues against the proposition that a single hotspot of the Hawaiian-Emperor type produced the Line Islands chain. Biochronologic data from the Line Islands indicate that the chain is not the temporal equivalent of the Emperor chain. Volcanic edifices of Cretaceous age are now known to extend from the Line Islands through the Mid-Pacific Mountains to the Marshall Islands and the western margin of the Pacific plate from Japan to the Marianas. A volcanic event occurred in the southern Line Islands during middle Eocene time; Eocene sediments were engulfed and altered by a volcanic eruption. The occurrence of both Cretaceous and Eocene volcanism in the southern Line Islands indicates that the history of the Line Islands is similar to that of the Marshall Islands.

Drilling results from mid-Pacific atoll and archipelagic apron sites in the Line Islands and Marshall Islands provinces lead to the conclusion that Oligocene sea-level falls detected in Atlantic passive margin sequences are also recorded in a mid-plate-tectonic setting in the Pacific Basin. The mid-Pacific sea-level falls are recorded by (a) the presence of distinct, coarse-grained, graded beds of turbidite origin, rich in reef-derived skeletal debris of Oligocene, Eocene, and Cretaceous age, that were redeposited in deep-water archipelagic apron carbonate sequences of middle and late Oligocene age now flanking the atolls and (b) a marked stratigraphic hiatus and solution unconformity in the subsurface of Enewetak atoll which dates an Oligocene period of atoll emergence correlative with both the deposition of the turbidites and the coastal offlap events discerned in Atlantic passive margins. Correlation of the subsidence path of Enewetak atoll with the development of the Oligocene solution unconformity shows that ca. 30 Ma sea level was as much as 100 m lower than at present.

A period of atoll emergence results in the development of a karst surface on the atoll. Upon resubmergence, this surface is preserved as a subsurface solution unconformity that can be identified by petrographic characteristics and a distinctive stable isotope signature imprinted by subaerial or phreatic diagenesis. Determination of the magnitude of Miocene sea-level falls by considering subsidence rates and depths to solution unconformities must take into account the lowering of the emergent atoll surface by dissolution. Previously, the depth to a Miocene solution unconformity below Midway Atoll was assumed to record a maximum highstand of sea level that preceded a period of island emergence. However, because of surface dissolution, the solution unconformity must represent some elevation between a past highstand and a subsequent lowstand. The subsurface stratigraphy of Midway Atoll shows that a late Miocene sea-level fall of 75–125 m below present sea level took place. The magnitude of this fall is greater than that previously determined, on the basis of stable isotope data, from Midway Atoll but approximates Miocene sea-level falls determined by seismic stratigraphic methods.