Sites 1196 and 1199 were both drilled into the top of the southern carbonate platform on the Marion Plateau. Site 1196 (proposed Site CS-06A) was drilled in 304 m of water, 20 km east of the Great Barrier Reef margin (Fig. F1). The site is positioned at the intersection of regional seismic line MAR07 (SP 2808) (Fig. F2) and local grid line MAR70 (SP 178). Two holes were drilled (Hole 1196A: 0-672.2 meters below seafloor [mbsf]; and Hole 1196B: 0-265.3 mbsf) through a 663-m sequence of carbonate platform sediments overlying a phosphate-rich substrate. One hole was drilled at Site 1199 (proposed Site CS-16A; 0-419.5 mbsf). This site is located in 315.7 m of water ~5 km northeast of Site 1196 (Fig. F1). The site is positioned on regional seismic line MAR20 (SP 2690).
Sites 1196 and 1199 provide information on the initiation and facies development of the Southern Marion Platform (SMP), the nature of unconformities separating each platform phase, and the age and nature of the section equivalent to the northern platform drilled at Site 1193 (Fig. F7 in the "Leg Summary" chapter). Postcruise analysis of recovered samples will enable the study of fluid flow processes within the carbonate platform as interpreted from the pore waters from adjacent slope sites (Sites 1197 and 1198).
Scattering of seismic energy within the well-indurated southern carbonate platform makes it difficult to determine seismically the stratigraphy and nature of sediment deposition underneath the platform cap. Despite this, analysis of regional seismic data suggested that Sites 1196 and 1199 should penetrate a thick sequence of late Miocene-Pliocene carbonate platform sediments representing the deposition of the youngest phase of carbonate platform growth observed on the Marion Plateau. The base of this sequence was predicted to overlie basinal sediments shed from the northern platform drilled at Site 1193. Drilling at Sites 1196 and 1199 only partially confirmed this interpretation. Although not conclusive, evidence from these sites suggests that only the upper 130 to 150 m of the platform facies could have accreted during the late Miocene, overturning the working hypothesis that this platform was completely of late Miocene to early Pliocene age. This discovery also necessitates a lower-amplitude sea level change for this time interval than was originally predicted.
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