GEOLOGICAL BACKGROUND

Volcanic rifted margins are the most common style of divergent margin in the Atlantic and, perhaps, worldwide. The Southeast Greenland margin is a type example of a volcanic rifted margin and is characterized by a broad seaward-dipping reflector sequence (SDRS) that onlaps continental (mainly Precambrian) crust to the west and terminates eastward in oceanic crust of early Tertiary age (Larsen, Saunders, Clift, et al., 1994). The rifted structure of the Southeast Greenland margin is structurally simple, with a well-understood plate kinematic history (e.g., Srivastava, 1978). Breakup took place within cratonic lithosphere, forming two conjugate margins, Southeast Greenland and the Rockall-Hatton margin.

The formation of large igneous provinces, such as the SDRS of the Southeast Greenland margin, results from the extrusion of vast volumes of mantle-derived magmas in a limited geographic area within a short time period. The geologically rapid eruption of large volumes of subaerial magmas and subsequent submergence of these basalts during continental rifting may result in significant, and previously unconstrained, chemical fluxes into the oceans, caused by seawater alteration. Magmatic structures, the absence of pillow lavas, and highly oxidized flow tops attest to the subaerial eruptive nature of the Southeast Greenland margin lavas. Additionally, the presence of subhorizontal geopetal structures within half-filled amygdules in lavas recovered on Leg 152, which presently dip at ~25º, suggest that alteration has continued after regional deformation and submergence of the lava pile (Larsen, Saunders, Clift, et al., 1994).

To date, there has been no quantitative investigation of the alteration of a submerged large igneous province, and the potential chemical fluxes related to the weathering of these phenomena remain unconstrained. Basaltic rocks from two holes (Holes 917A and 918D) occupied during Leg 152 have been described, quantitative records of the bulk-rock alteration have been made, and the type and distribution of veins and vesicle fillings have been established. These data provide a unique database for the calculation of the chemical fluxes associated with the submarine alteration of a large igneous province.

Summary of Hole 917A

Hole 917A is located at a depth of 508 m, ~50 km from the coast. It is the most landward site in the 6-km-long transect that was drilled during Leg 152, across the central part of the East Greenland Shelf at ~63ºN. Hole 917A was penetrated at the featheredge of the SDRS into metasedimentary rocks beneath the SDRS (Larsen, Saunders, Clift, et al., 1994).

Six lithologic units were found in Hole 917A, with a total penetration of 874.9 m. A thin layer (~28.7 m) of Quaternary glaciomarine silt with dropstones (Unit I) unconformably overlies upper middle Eocene marine micaceous sandy-siltstones (Unit II; 28.7-37.7 mbsf) with abundant fragments of volcanic glass and basalt. A basal volcaniclastic conglomerate (Unit III; 37.7-41.9 mbsf) separates Unit II and the basaltic flows (Unit IV), which were intersected at 41.9 mbsf.

Approximately 780 m of basalt (Unit IV), which composes the SDRS, was penetrated in Hole 917A. Ninety-one flows and one intrusive sheet were recognized from the recovered core. The volcanic succession is divided into three series: an Upper Series (41.9-183.4 mbsf) of predominantly olivine basalts and picrites; the Middle Series (184.1-376.7 mbsf) of more evolved basalts and dacites with minor pyroclastic units; and the Lower Series (376.7-821.06 mbsf) of basalts and uncommon olivine basalts. The Upper and Middle Series are separated by a thin (~1 m) fluvial sandstone. Based on the presence of red oxidized flow tops and rare reddened soil horizons, the entire sequence appears to have been erupted subaerially (Larsen, Saunders, Clift, et al., 1994).

The lava sequences are underlain by coarse-grained quartzose sandstone (Unit V; 821.06-821.20 mbsf) of unknown age. Steeply dipping (60º) carbonaceous mudstones and sandstones (Unit VI; 821.2-874.9 mbsf) underlie the quartzite and extend to the bottom of the hole. The age and affinities of these sediments are unknown (Larsen, Saunders, Clift, et al., 1994).

Summary of Hole 918D

Four holes were drilled at Site 918, near the center of the SDRS on the upper continental rise, ~130 km from the Southeast Greenland coastline. The site is located in 1868.56 m of water. Penetration of Hole 918D was through ~1200 m of sediment, subdivided into five lithologic units, before reaching basaltic basement at 1189.4 mbsf (Larsen, Saunders, Clift, et al., 1994).

Lithologic Unit I (0-600 mbsf) comprises Miocene to Holocene dark gray silt with both volcanogenic and continentally derived components. This is underlain by 288.5 m of heavily burrowed, upper Miocene to lower Miocene nannofossil chalk and silt (600-806.5 mbsf). Lower Miocene to upper Oligocene sands, silts, and nannofossil chalk compose Unit III (806.5-1108.2 mbsf), and these unconformably overlie interbedded middle to lower Eocene nannofossil chalk and volcaniclastic silt (Unit IV; 1108.2-1157.9 mbsf). Unit V (1157.9-1189.4 mbsf) consists of lower Eocene glauconitic sandy silt interbedded with calcareous sand.

The first basaltic rocks were found at 1162.2 mbsf (Core 152-918D-93R), and basaltic basement was reached at 1189.4 mbsf. Highly altered rocks, strongly weathered and oxidized by subaerial processes, compose the upper 18 m of the basaltic basement. The first rocks preserving any primary minerals were recovered from 1206.0 mbsf (Core 152-918D-98R-2, 65 cm), and recovery of these rocks continued to the bottom of Hole 918D (1302.3 mbsf). Eighteen basaltic flows were identified (Larsen, Saunders, Clift, et al., 1994).

NEXT