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doi:10.14379/iodp.sp.359add.2015

Scientific objectives

Proposed Site MAL-02A (primary site)

Proposed Site MAL-02A is located at 4.933156°N, 73.027983°E, approximately 1 nmi east of proposed Site MAL-01A in the western part of Kardiva Channel at a water depth of 516 m (Figs. F1, F2). The sediment is periplatform ooze. The seafloor is an overall flat surface covered by 400 m wide and 4 m high submarine mud waves. The site is selected just basinward of the first onlap of the oldest drift deposits (DS-01) onto the toe of slope of the drowned bank (top Platform Sequence 10) (Fig. F3). Here, DS-01 reaches its greatest thickness. Therefore, this site is expected to record a most complete predrowning–drowning–postdrowning succession, allowing reconstruction of environmental changes at this turnover.

The site is positioned to recover the sedimentary succession deposited during the late-stage evolution of the middle Miocene drowned carbonate bank and the onset of drift deposition. The specific objectives at this site are

1. To provide a detailed reconstruction of the predrowning, drowning, and postdrowning evolution of the carbonate bank by linking the seismic stratigraphic record to the sedimentary record;
2. To constrain the timing of this evolution, thus allowing age assignments of unconformities, sedimentary interruptions, sedimentary turnovers, and onset of drift deposition; and
3. To reconstruct and date the bank to drift turnover.

Deepening proposed Site MAL-02A will allow recovery of the Oligocene/Miocene boundary, which is calculated to be at 978 mbsf. The expected nature of the succession down to this boundary is chalk. An additional 30 m should be drilled for the logging rathole. The succession is characterized by a downhole increase in lithification from ooze at the top to chalk at the base (Fig. F4). A deeper penetration depth at proposed Site MAL-02A (to 1010 mbsf) would also help to ensure good logging results from the lower slope deposits of the drowned carbonate platform (PS-01 to PS-10). This is important, as the ooze, chalk, and limestone alternation at proposed Site MAL-01A could possibly create problems with regard to hole quality.

Proposed Site MAL-08A (new alternate site)

Proposed Site MAL-08A is located at 4.90693350°N, 73.00819003°E, in the westernmost part of the northern transect. It is located at the western margin of the Inner Sea in the western part of Kardiva Channel at a water depth of 432 m (Fig. F2). Inner Sea sediments are hemipelagic carbonate ooze consisting of an admixture of components exported from the atolls and pelagic components (periplatform ooze). The seafloor is an overall flat surface covered by 400 m wide and 4 m high submarine mud waves. The site is located at a key position to document and reconstruct the carbonate bank depositional system of the drowned Miocene bank and to link the seismic sequences to facies; here, a drowning unconformity does not mask underlying geometries, which is rare (Figs. F5, F6).

Proposed Site MAL-08A is positioned to recover the sedimentary succession deposited during the late-stage evolution of the drowned middle Miocene bank and to characterize the onset of drift sedimentation. The specific objectives are

1. To provide a detailed reconstruction of the predrowning, drowning, and postdrowning evolution of the carbonate bank by linking the seismic stratigraphic record to the sedimentary record (i.e., depositional facies);
2. To constrain the timing of this evolution, thus allowing age assignments of unconformities, sedimentary interruptions, sedimentary turnovers, and onset of drift deposition; and
3. To reconstruct and date the bank to drift turnover.

The target horizon at proposed Site MAL-08A is the Oligocene/Miocene boundary, which lies at the proposed depth of penetration of 1100 mbsf, including a rathole for logging. The upper part of the succession is characterized by carbonate ooze overlying the drowning unconformity. The nature of the unconformity is unclear. No strong lithification is expected, as impedance contrast is not very high and no acoustic blanking occurs below the unconformity (Fig. F7). The Platform Sequences 09 and 10, delimited by Sequence Boundaries (SB) 09 and 10, are expected to consist of chalk to limestone (proximal carbonate bank slope deposits). Downhole lithification is first expected to decrease because more distal slope deposits will be drilled. These deposits have lower aragonite contents because the source of neritic input was located further west. Therefore, the diagenetic potential of this sediment is reduced compared to the overlying, more proximal slope deposits. Degree of lithification toward the base of the succession is finally expected to increase, and limestone to chalk lithologies are expected in the lower interval of the site.

Results of existing nearby drilling sites

The sedimentary succession at nearby Ocean Drilling Program (ODP) Site 716 mostly consists of carbonate ooze composed of planktonic foraminifers, bioclasts, pteropods, and minor amounts of benthic foraminifers, changing downhole into ooze to chalk. It shows distinct changes of the amount of fine-grained (<63 µm) material (Fig. F8). Data for the fine fraction content of Site 716 is combined from different sources. To 65.05 mbsf, the core was newly sampled and the grain-size parameter measured at a 10 cm interval. Analysis of the interval between 32.29 and 27.63 mbsf has been done on data from Hole 716A. Between 264.4 and 147.4 mbsf, the fine fraction amount varies between 48% and 97% with an overall upcore decrease. In this interval, a trend of overall increase of grain size is superimposed on fluctuations of around 5% in the lower part and 40% in the upper part. Between 147.4 mbsf and the seafloor, several sediment packages occur, characterized by systematic trends of changes in the amount of fine fraction. The lower package is between 147.4 and 87.0 mbsf. At 28.73 mbsf, the amount of the fine fraction increases to 10.5 mbsf. The remaining interval is characterized by three coarsening-upward packages.

Proposed Site MAL-09A (new primary site)

Proposed Site MAL-09A lies east of the northern and southern transect. It is located to recover the entire drift succession (DS01–DS09). This site was selected to replace previous primary Site MAL-04B (Betzler et al., 2014), which is believed to have a mass flow intercalated into the succession at a depth of 720 ms between 63250 and 69000, based on analysis of a new parasound line recovered crossing the site. The specific objectives for the new primary site are

1. To analyze cyclostratigraphy of drift deposits, therefore providing reconstructions of changes in the current regime and monsoon cyclicity; and
2. To constrain the timing of unconformities and sedimentary interruptions.

Proposed Site MAL-09A is located at 4.85025773°N, 73.28367339°E, approximately 9.1 km south of Site 716/proposed Site MAL-04B in the Inner Sea part of Kardiva Channel (Figs. F1, F9) in a water depth of 488 m. Proposed penetration is 714 mbsf. The sediment is periplatform ooze. The seafloor is flat overall. The site is located to recover a complete distal succession of DS-01 to DS-09 (Figs. F10, F11). It is located near the deepest and oldest gateway of the Maldives, the northeastern Kardiva Channel. This will allow us to characterize the onset of the current-controlled depositional regime and to date the reorganizations of the system indicated by the occurrence of SBs DS-02–DS09.

The target horizon at proposed Site MAL-09A is the basal SB of PS-09, which was formed during the middle Miocene. The model for subsurface velocity variations is shown in Figure F12 and shows that the proposed depth of PS-09 is 663 mbsf. An additional 50 m should be drilled for the logging rathole, resulting in a total depth of penetration of 713 mbsf. The succession is characterized by a downhole increase in lithification from ooze at the top to chalk at the base.

Results of existing nearby drilling sites

The carbonate ooze succession drilled at Site 716 is presented in Figure F8.

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