Janet A Haggerty

Evidence from fluid seeps atop serpentine seamounts in the Mariana forearc: Clues for emplacement of the seamounts and their relationship to forearc tectonics

Abstract The Mariana forearc is a non-accretionary forearc with numerous seamounts. Dredge hauls from these seamounts are composed primarily of serpentinite, indicating a non-volcanic origin. In addition, dredged carbonate samples reveal (1) the presence of aragonite in water depths greater than the aragonite compensation depth, (2) unusual porosity and cement types, (3) light carbon and heavy oxygen isotopic compositions, and (4) an unusual trace element geochemistry of aragonite and calcite. These mineralogic, petrologic, and geochemical characteristics formed the basis of a prediction that methane-bearing fluids are venting from these seamounts. Subsequent to this prediction, submersible dives on a Mariana forearc seamount found and sampled chimneys seeping cool water that contains dissolved methane. The serpentine substrate beneath the chimneys appears to form from cold gravitational flows of serpentine material. Fluid inclusion contents of chimney samples also show an earlier history of methane, as well as longer chain hydrocarbons, aromatics and acetate ions. Drilling into the flank and the summit of this seamount by the Ocean Drilling Program during Leg 125 confirmed the presence of methane, other light alkanes, and organic acids in the present-day interstitial pore fluid of the cores. The presence of aromatic compounds and organic acids, if derived from the maturation of organic compounds, indicates a thermogenic origin for these volatiles. Strontium isotopic ratios of the aragonitic chimney samples and of the present-day interstitial waters range from present-day seawater values to significantly lower values. The strontium isotopic ratios of the pore waters within the seamount trend toward a relatively less radiogenic composition with increasing depth below the seafloor. This strontium isotopic signature is probably not the consequence of near-surface processes, but more likely a contribution from an igneous source deeper in the lithosphere. The lack of a large accretionary prism in the Mariana forearc removes the complication of fluid contribution from dewatering of the prism by compression during convergence, as in the Oregon-Washington or Barbados margins. The organic compounds in the fluid inclusions and interstitial pore fluids, as well as the isotopic and the geochemical composition of the carbonates and the dissolved constituents, suggest that fluids are derived from dewatering, compression, and dehydration of subducted sediments and the subducting plate. Furthermore, the composition of the fluid is influenced by thermal maturation of organics in subducted sediments as well as by a contribution from a deep igneous source.

Late Cretaceous and Eocene volcanism in the southern Line Islands and implications for hotspot theory

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.

Deep-sea drilling on the upper continental rise off New Jersey, DSDP Sites 604 and 605

Deep Sea Drilling Project Sites 604 and 60S on the upper continental rise are the first of a series of cored holes along the “New Jersey transect” which, when completed, will provide the first comprehensive dipwise suite of drill holes across a passive margin from the coastal plain to the abyssal plain. Our drilling results document the age of important seismic sequence boundaries and allow their correlation with wells on the continental shelf and slope as well as with the regional oceanic seismic stratigraphy. Hole 605,156 km (97 mi) southeast of Atlantic City, New Jersey, and drilled 816.7 m down to mid-Maestrichtian limestones, penetrated a near-complete Cretaceous/Tertiary boundary section overlain by a 200-m expanded Paleocene sequence. Unusually high amounts of terrigenous silts and glauconite are present at the boundary and immediately above. Among the several hypotheses discussed, we suggest that the terrigenous silts and glauconite may represent a high-energy event such as a tsunami caused by a Cretaceous/Tertiary impact. Site 604, 5 km (3 mi) seaward of Site 605, was terminated in upper Miocene glauconitic sands and debris flows at 294.5 m by unstable hole conditions. These sediments contain shelf-derived gravels and exotic blocks of Eocene chalk (up to 50 cm across) eroded from bedrock that is today widely exposed on the adjacent slope. Our drilling results show that denudation of the Eocene units was not limited to the Oligocene A u erosional event, but that major loss occurred during late Miocene and later glacial sea-level lowstands.

DSDP Site 603: First deep (>1000-m) penetration of the continental rise along the passive margin of eastern North America

Drilling at Deep Sea Drilling Project Site 603 has provided the first deep (>1000-m) penetration of strata beneath the continental rise off the Atlantic margin of North America. Nearly continuously cored through 1585 m of section down to Berriasian pelagic limestones, the site 435 km (270 mi) east of Cape Hatteras intersected an extensive Lower Cretaceous deep-sea fan complex, which provides new information on the petroleum potential of the continental rise. Hauterivian to early Aptian in age, this 208-m interval of interbedded limestones, sand, and black shale turbidites begs the existence of any post-Valanginian reefs along the Baltimore Canyon Trough. Less extensive terrigenous turbidites were encountered higher in the section up to the Cretaceous/Tertiary boundary, which is marked by a current-laminated sand rich in dark spherules. Pelagic early Paleogene clays are disconformably overlain by Miocene pelagic mud. Turbiditic silts and clays began to accumulate rapidly at this site during the middle Miocene, leading to deposition of muddy contourites that formed the Lower Continental Rise Hills of the Hatteras Outer Ridge as sand turbidites were ponded concurrently on its landward side. The section at Site 603 confirms the concept that eustatic and other large-scale events subdivide Earth history into distinct chapters allowing the correlation of deep-sea seismic sequence boundaries with continental shelf and margin unconformities.

Mesozoic-Cenozoic clastic depositional environments revealed by DSDP Leg 93 drilling on the continental rise off the eastern United States

Summary Prior to Deep Sea Drilling Project (DSDP) Leg 93 (1983), drill data along the continental rise of the Atlantic margin of the United States were quite limited compared to those of the adjacent continental shelf and the deeper, more seaward expanses of the North American Basin. Interpretations of the geologic history and of processes that controlled sedimentation along the rise were strongly dependent on studies of seismic reflection profiles. DSDP Leg 93 drilled deep holes on both the lower and upper rise, allowing correlation with commercial wells on land and offshore (as well as with subsequent DSDP Leg 95 holes along the ‘New Jersey Transect’) and providing the first down dip suite of drill holes across a passive continental margin from the coastal plain to the abyssal plain. Site 603 on the lower rise 270 miles east of Cape Hatteras was cored nearly continuously over 1585 m to Berriasian pelagic limestones. It intersected an extensive Lower Cretaceous deep-sea fan complex which provides new information on the petroleum potential of the rise. Hauterivian to early Aptian in age, this 208 m interval of interbedded limestones, sand and blackshale turbidites calls into question the existence of any post Valanginian reefs along the Baltimore Canyon Trough. Less extensive terrigenous turbidites were encountered as far up in the section as the Cretaceous-Tertiary (K-T) boundary. The K-T boundary is marked by a current-laminated sand rich in dark, 1 mm diameter spherules which may denote an extraterrestrial impact event. DSDP Sites 604 and 605 on the upper rise, the first along the ‘New Jersey Transect’, are located some 100 miles south-east of Atlantic City, New Jersey. Hole 605, drilled 816.7 m down to mid-Maestrichtian limestones, penetrated a near complete Cretaceous-Tertiary boundary section, above which 20 m of lower Palaeocene are separated by a disconformity from an expanded 175 m Palaeocene sequence. Terrigenous silts and glauconite at the K-T boundary and immediately above suggest either significant sea-level change, increased current erosion along the adjacent shelf and slope, increased terrigenous input caused by decreased vegetation, a high energy event (tsunami?), or some combination of these possible factors. Site 604, 3 miles seaward of Site 605, was terminated by unstable hole conditions at 294.5 m within a unit of Miocene glauconitic sands and debris flows. Emplaced largely during the Tortonian (8.2–10.0 Ma; Vail cycle TM3.1), these upper Miocene sediments contain shelf-derived gravels, exotic blocks of Eocene chalk (up to 50 cm across) eroded from the adjacent slope, and clasts of middle and upper Miocene carbonates or silts derived from canyon walls or shallow water strata upslope. Study of closely spaced, high resolution seismic profiles suggests that large-scale regional erosion (canyon cutting), which is related to the debris flows, began during the late middle Miocene. On the lower rise, turbiditic silts and clays began to accumulate rapidly during the middle Miocene. Under the influence of a strengthening Western Boundary Undercurrent, these were deposited as muddy contourites in antidune-like sediment waves which, at site 603, grew rapidly with no appreciable break in sedimentation until at least early Pleistocene times to form the present Lower Continental Rise Hills of the Hatteras Outer Ridge (HOR). The somewhat elevated edge of the Lower Continental Rise Terrace formed as a natural levee behind which the coarser portions of the terrigenous turbidites were ponded to form the terrace. No coarse clastics that bypassed the pond were deposited with the clays of the HOR at this locality. Throughout the study, seismic sequence boundaries of the upper and lower continental rise were calibrated and correlated with continental margin unconformities as well as with deep sea reflection horizons.