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Seismic Stratigraphic Results of Leg 194

Drilling at all Leg 194 primary sites and two alternate sites penetrated and sampled a Neogene sedimentary record that can be used to calibrate the sequence stratigraphic architecture of the Marion Plateau megasequences. The recovered sediments calibrate the regional seismic stratigraphy by identifying the lithologic signature and providing the chronostratigraphic framework for the seismic megasequences, thus constraining the carbonate platform history and the magnitude and timing of sea level changes on the Marion Plateau.

Time-Depth Correlation

In order to correlate core and log information accurately with the seismic data, a time-depth curve was constructed for each site (Fig. F10). These curves were obtained by integrating velocity data from sonic logs and shipboard velocity measurements and by calibrating these curves with results from check-shot surveys. Check-shot surveys could only be performed at Sites 1194, 1195, and 1196. For the remaining sites, virtual check shots were defined that link prominent high-amplitude reflections to unique horizons in the cores. These horizons are usually hardgrounds or exposure surfaces at megasequence boundaries. Near the bottom of the drill holes, the basement reflection was used as a fixed point to tie the critical lower part of the time-depth curve. Variations in the slopes of the time-depth curve reflect the high velocities in the platform sections (Sites 1193, 1196, and 1199), the medium velocities in the proximal slope sections (Sites 1194, 1197, and 1198) and the lowest velocities in the most distal locations (Sites 1192 and 1195) (Fig. F10). Time-depth curves, combined with the age model curves (Fig. F11), were used to assign chronostratigraphic datums to the seismic sequence boundaries at each site, which then can be compared across the transect (Fig. F12A, F12B). The age models are mostly based on core catcher analyses and therefore have limited depth resolution. An overall good match between the ages of the sequence boundaries at the different sites, within limits of seismic and shipboard biostratigraphic resolution, confirms the concept of seismic stratigraphic correlations, in which seismic reflections have a chronostratigraphic significance. Age differences at boundaries between sites may be reduced once a more refined chronostratigraphy is established postcruise. The following sections discuss the major findings for each of the megasequences. The ages and depths of seismic megasequence boundaries as well as lithologic unit boundaries at each site are plotted in Figure F12A and F12B.

Megasequence D

At all Leg 194 sites, Megasequence D, the youngest of the Marion Plateau megasequences, is composed of hemipelagic drift deposits. Megasequence D is only missing at the top of the southern MP2/MP3 platform edifice (Sites 1196 and 1999) where Megasequence C outcrops at the seafloor. The base of Megasequence D is a regional unconformity that is observed at all proximal platform sites (1193, 1197, and 1198) characterized by a hiatus lined with submarine hardgrounds, which provides an excellent link between seismic unconformities, biostratigraphic hiatuses, and nondepositional processes on the seafloor. At these sites, the age of the onset of Megasequence D sedimentation is dependent on the sites' locations relative to the depocenters of drift deposition. These ages are always younger than the ages of the base of Megasequence D in the distal areas where the sequence boundary is conformable. Sites 1192, 1194, and 1195 were drilled into the conformable succession at the base of Megasequence D, recovering the oldest sediments within this megasequence. At these sites, the base of Megasequence D was dated at 7.2 (Site 1194), and 7.2 F12A). Sites 1192 and 1195 are located in the most distal locations and have a well-constrained age model curve, so a basal age of ~7 Ma for Megasequence D is likely. Two smaller-scale seismic unconformities that represent current-controlled drift unconformities reflecting lateral shifts of depocenters and local hiatuses can be traced throughout the seismic grid. Their age range along the transect is 2.5-3.0 and 5.0-5.5 Ma, respectively (Fig. F12).

The C/D boundary at either side of the southern MP2/MP3 platform coincides with the end of platform-derived shedding of neritic constituents onto the slopes (Fig. F12B). The age of ~7.2 Ma is thus assumed to reflect the end of the MP3 carbonate platform growth phase in this area and may date the drowning of MP3 at least 1 m.y. earlier than originally predicted (Pigram, 1993).

Megasequence C

Megasequence C is best developed near the MP2/MP3 carbonate edifice, where it records the platform-derived sedimentation of the MP3 growth phase (Fig. F12B). The platform proximal sediments were drilled at Sites 1197 and 1198, and despite very low recovery, their age and platform-derived constituents indicate the existence of a late Miocene MP3 platform. Megasequence C at Site 1197 is characterized by relatively fine to medium grainstones (Unit II). At Site 1198, Megasequence C was deposited at the base of an escarpment and consists of coarse rudstone and floatstone (Subunits IIA and IIB) in the lower part of the section. Further from the platform, these proximal periplatform sediments interfinger with time-equivalent hemipelagic drift deposits. Based on the downlap of these Megasequence C drift deposits northwest of Site 1198 onto slope deposits of underlying Megasequence B, the southern platform edifice was believed to be entirely of late Miocene age. Drilling on the platform itself (Sites 1196 and 1199) revealed, however, that only the uppermost 100-180 m potentially consist of a MP3 phase (lithologic Subunits IA-IC). Biostratigraphic dating below this depth gave a middle Miocene age. A low-amplitude, low-frequency reflection underneath the MP3 top is a candidate for the MP2/MP3 boundary (Fig. F12B). This interval between the seafloor and ~110-130 mbsf dips gently to the southeast and coincides approximately with Megasequence C sediments on the downcurrent slope at Site 1197. The age models for the B/C boundary indicate ages of 11.9 (Site 1198), and ~10 best seismic coherency and age control, an age of 11.0 boundary. This boundary correlates to the karstic top of the northern MP2 platform, placing an upper age limit on MP2 platform growth.

Megasequence B

In the northwestern parts of the MP2 platform, such as at Site 1193, Megasequence B consists, at the base, of inclined slope deposits that can be seen on seismic data as inclined reflections underneath the MP2 platform (Fig. F12A). At Sites 1193 and 1194, these early Miocene slope deposits were composed mostly of fine silt-sized carbonate debris, which became mixed with a pelagic fraction in a periplatform environment (lithologic Units IV and V at both sites). The top of these periplatform sediments immediately underneath the platform section at Site 1193 could be confined to ~16 Ma. No age-diagnostic markers were found in the platform section above. Considering that the top of Megasequence B (platform top) was dated through seismic correlation along the B/C boundary to 11.0 (+/- 0.2) Ma, MP2 may span about 5 m.y. in the middle Miocene. The top of the MP2 platform at Site 1193 shows signs of meteoric diagenesis. Topographically below the MP2 platform, Site 1194 penetrated an upper-slope section in a margin proximal position adjacent to MP2. The top of Megasequence B at Site 1194 is represented by a hardground surface, that caps an interval interpreted as a neritic outer ramp deposited in <60 m of paleowater depth (lithologic subunit IIIA). The base of this interval, marked by another hardground surface, can be clearly mapped on the seismic data and correlates to an age in the open plateau area of 12.5 Ma. The interval overlies neritic upper-slope and hemipelagic sediments, indicating a shallowing-upward trend and thus a sea level lowering in the latest middle Miocene.

Megasequence B on the open plateau is mostly composed of a mixture of distal periplatform and pelagic components. In the most distal Site 1195, the top of Megasequence B coincides with a 20- to 30-m-thick interval rich in glauconites overlying distal periplatform sediments. The absence of neritic components indicates a reduction of neritic carbonate production at the end of the middle Miocene.

At Site 1198, situated adjacent to the escarpment in a proximal position to the southern MP2/MP3 platform, the top 70 m of Megasequence B (lithologic Subunit IIC) thicken toward the MP2/MP3 platform escarpment, documenting shedding of neritic material at the end of the middle Miocene. These periplatform sediments overlie hemipelagic deposits (lithologic Unit III). At Sites 1196 and 1199 on the platform, the >500-m-thick platform sequence below the presumed B/C boundary cannot be further subdivided because of the transparent seismic facies. It is likely that after its initiation onto a substrate of latest Oligocene age, this southern platform complex was a product of several growth phases in the early and middle Miocene. Interestingly, at both sites on the slope (Site 1197 and 1198), no early Miocene platform shedding is indicated, neither by debris in the periplatform sediments, nor geometrically by a thickening toward the platform. This observation might be related to the strong influence of currents from north to south that shape the geometry of the platform. The paleo platform shape can be estimated by tracking the weak reflection assumed to be the top of Megasequence B (equivalent to the top of the MP2 platform) at a subsurface depth of ~100-130 mbsf. This reflection can be partly traced on the seismic data and displays a topography with a rim at the northwest, upcurrent escarpment and a gentle dip toward the southeastern, downcurrent slope at Site 1197. At Site 1197 the B/C boundary caps the coarsest interval at that site (lithologic Unit III) and forms a thick prograding slope unit that forms a downcore transition from steeper-dipping slope to almost flat hemipelagic deposits (lithologic Units III-V).

Megasequence A and Basement

In the northern survey area, Megasequence A consists of a siliciclastic substrate that directly overlies acoustic basement. Megasequence A has a highly variable thickness, infilling small-scale basement irregularities. Only Sites 1193 and 1195 were characterized as having siliciclastic sediments directly overlaying basement. Thus, Megasequence A is not easily traced through all Leg 194 sites. At Sites 1195 and 1198, the basement was overlain by a thin veneer of coarse carbonate deposits of undefined age. These sediments at Site 1198 can be seen on the seismic data as a thin onlapping unit directly overlying the basement reflection. The acoustic basement itself is highly variable, as shown by its different seismic signature across the study area. At Leg 194 sites, it consists of altered basalt flows and volcaniclastic breccias/conglomerates.

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