A complete description of drilling at Site 989 is provided in the Leg 163 Initial Reports (Duncan, Larsen, Allan, et al., 1996). The site was located 43 km east of the Greenland coast, within the southern drilling transect of Leg 163 (Fig. 1). Based on results from Leg 152 (Larsen, Saunders, Cliff, et al., 1994), Site 989 was selected to examine the landward, and presumably oldest, featheredge of the SDRS sequence and to evaluate the nature of continental breakup in this region (Fig. 2). Tegner and Duncan (Chap. 6, this volume) report a mean ⁴⁰Ar-³⁹Ar age of 57.1 ± 1.3 Ma for lavas at Site 989. The uppermost igneous flow unit (Unit 1) is normally magnetized (Duncan, Larsen, Allan, et al., 1996) and can be correlated with Chron C25n, while the underlying flow unit (Unit 2) is reversely magnetized corresponding to Chron C25r. Tegner and Duncan (Chap. 6, this volume) surmise that the lavas at Site 989 are time equivalents to lavas with similar trace element-depleted Icelandic compositions recovered at Sites 915 and 990 (Duncan, Larsen, Allan, et al., 1996). The lavas from Site 989 are also similar in age to lavas of the main phase of flood basalt volcanism in central East Greenland (Storey et al., 1996).

Unit 1 recovered at Site 989 is extraordinarily thick (>69 m). Shipboard analysis (Duncan, Larsen, Allan, et al., 1996) shows it to be composed of numerous individual flows 0.1-10 m thick, separated by finely crystalline flow tops (Fig. 3, Fig. 4; see below). The unit is described as an aphyric basalt with rare plagioclase glomerocrysts ranging in size from 0.5 to 3.0 mm. Plagioclase glomerocrysts are restricted to the interior of flows and are absent from quench zones. The groundmass contains 60%-90% plagioclase and augite, in subequal proportions, 40%-10% glass (mostly altered to clay), and small amounts (1%-4%) of magnetite. Olivine is present in trace amounts (1%), but is not ubiquitous.

Flow textures in Unit 1 are typically intersertal within individual flows, and variolitic in the finely crystalline boundaries between flows. The textural boundaries identified as the tops of individual flow units (see Fig. 3) are subhorizontal and are marked by a 1-2 cm band of finely crystalline lava that commonly has a bimodal distribution of vesicle sizes (2-4 mm irregularly shaped vesicles surrounded by numerous, more spherical vesicles <100 µm in size). The upper boundary of this band is often diffuse and irregular, and the boundary is typically more macroscopically altered than the surrounding lava. Lava immediately below each flow top is often quite dense because of the absence of larger vesicles. Other textural features observed in this unit (but not mapped as flow tops) include centimeter-scale and finely crystalline bands that separate lavas of similar (rather than differing) vesicularity, macroscopic changes in vesicularity (from moderately vesicular to dense) that lack a finely crystalline band, irregular and thin (<0.5 mm) finely crystalline bands, typically containing large (2-4 mm) vesicles, and diffuse and patchy bands of finely crystalline lava, commonly associated with large (up to 5 mm) zeolite-filled vesicles.

The base of Unit 1 is marked by very finely crystalline lava containing pipe vesicles. In Hawaii, pipe-vesicle-bearing flows are limited to pahoehoe flows emplaced in regions of low slope (Wilmoth and Walker, 1993), suggesting similar topographic conditions existed at Site 989 during the emplacement of Unit 1. Although initially described as compound, the gradational nature of the flow boundaries (Fig. 3) suggests that Unit 1 may better fit Walker's (1972) description of a multiple flow, that is, a flow that, while comprised of numerous stacked flow lobes, cooled as a single unit.

Only 11 m of core were recovered from Unit 2. The top of the flow was not preserved, and the recovered material showed no evidence of increased oxidation or vesiculation approaching the top of the flow. The Shipboard Scientific Party (Duncan, Larsen, Allan, et al., 1996) estimated that 0.5 m of flow top was missing and concluded that eruptions for Units 1 and 2 were not separated appreciably in time. No internal flow boundaries, similar to those seen in Unit 1, were recognized in Unit 2, although the unit contains abundant segregation patches similar to those in flows identified as aa at Site 990.

The lavas of Unit 1 have distinctive textures in thin section characterized by intergrown plagioclase and pyroxene crystals radiating from individual nucleation points (Fig. 5). These intergrowths are most likely spherulitic in three dimensions, and are classified as intersertal based on both the crystal forms and the abundance of meso-stasis. These radiating crystal clumps are separated from each other by 20%-30% mesostasis (now clay) and vesicles. Solitary plagioclase laths are found throughout the mesostasis, suggesting that plagioclase was the first phase to nucleate.These laths are commonly 0.5-1 mm long, as are plagioclase laths in the radiating clusters. There are also occasional glomerocrystic clots of larger and more equant plagioclase crystals.

The flow textures just described are remarkably constant throughout the unit, except within 10 m of the flow base. Here similar radiating plagioclase-pyroxene intergrowths are present, but the individual crystals in the clusters are larger (up to 2 mm) and more equant than those found stratigraphically higher. The coarser grain size toward the base of the unit is characteristic of a thick (10.2 m) individual flow near the base of Unit 1 (Fig. 4). In contrast, a sample from the pipe-vesicle-bearing flow base is very fine grained (with a maximum crystal length of 0.1-0.2 mm), and lacks the prominent crystal clusters of the rest of the flow. This grain-size reduction likely reflects a relatively high rate of cooling, as would be expected along the base of the first flow of a multiple flow unit.

The finely crystalline bands (Fig. 3) also have textures that are both distinctive and similar regardless of their position in Unit 1. These zones usually have irregular upper boundaries that show an abrupt transition to the intersertal textures described above. The bands themselves are characterized by plumose intergrowths of plagioclase and pyroxene (Fig. 5b) and abundant mesostasis, especially in the immediate vicinity of the relatively abundant large vesicles. These finely crystalline zones grade downward into intersertal textures, with the transition occurring over <10 mm. Another distinctive feature of the bands is the prevalence of alteration minerals, especially in the larger vesicles (Fig. 3) (Duncan, Larsen, Allan, et al., 1996).