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Upwelling Systems: Evolution since the early miocene
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Upwelling Systems : Evolution Since the Early Miocene
Journal of Climate, McIntyre , A. Modern Pacific Coccolithophorida: A paleontological thermometer. II, The seasonally reversing Indian monsoon system influences the oceanographic and depositional processes Tomczak and Godfrey Southwestern winds prevail during Northern Hemisphere summer April—November while northeastern winds prevail during winter December—March.
These winds generate ocean currents that are directed westward in the winter and eastward in the summer. Inter-seasonally, a band of Indian Ocean equatorial westerlies are established, enforcing strong eastward-flowing surface currents with velocities up to 1.
The rectangles labeled B and C in a and b outline the detailed views shown in b and c , respectively. Stippled white line indicates position of seismic cross section shown in d. Satellite imagery and seafloor imagery from www. These were strategically positioned to core the most relevant intervals of the Neogene carbonate platform succession. The drift facies encompass all current-controlled sediments of the Maldives carbonate platform sheeted drifts and drift fans, submarine dunes.
During the early Eocene, carbonate production was established, when flat-topped carbonate banks originated on topographic highs created by the volcanic basement. During the late Oligocene, bank margins typically had elevated rims that separated bank-interior areas from the open ocean. During the early Miocene, carbonate production became restricted to narrow bands at the respective most oceanward areas, forming the double row of platforms.
During the Miocene, platform margins prograded towards the Inner Sea, as recognized in different versions of seismic reflection data, irrespective of seismic resolution Purdy and Bertram ; Aubert and Droxler ; Belopolsky and Droxler a , b ; Betzler et al. Coincident with the partial drowning of the Maldives carbonate bank, drift deposits occupied the Inner Sea and the flanks of the atolls Betzler et al. The current control on the platform evolution continues until today and can be linked to the Indian Monsoon circulation Betzler et al.
Since the inception of the currents, two additional drowning steps have affected the Maldives Betzler et al. Flat-topped and atoll-shaped banks are interpreted to have drowned quickly, whereas mound-shaped banks are interpreted to have undergone sequential drowning under elevated nutrient fluxes Betzler et al. The data consist of two main packages: 1 a set of seismic data from various vintages Betzler et al.
In addition, published low- to medium-resolution industrial reflection seismic lines shot for Elf in — Purdy and Bertram ; Aubert and Droxler and for Shell in — Belopolsky and Droxler a , b were used for regional context and initial sequence analysis. Interpretation and visualization was performed on time-migrated data in the time domain using the software package Petrel Schlumberger.
All seismic data were tied to the seafloor topography computed from the multibeam data acquired during the site survey. Core retrieval, handling and all on board measurements including downhole logging are described in detail in the methods section of the Expedition Proceedings Volume Betzler et al. Seismic line across the Maldives Inner Sea for location of line see Fig.
Ages of selected horizons are given. Numbers 1—6 refer to the turning points of the platform evolution discussed in the text. Platform deposits and drift deposits are seismically well-resolved and are based upon their differing geometries, with a convex down shape characteristic of the platform clinoforms and a convex up shape characteristic of the drift deposits. Note that the position of Site U is projected onto the seismic line.
See text for details.uqenagog.cf
Carbonate mineralogy: blue : aragonite, green : high Mg calcite, gray : low Mg calcite, red : dolomite. For the calcareous nannoplankton, the standard zonal scheme of Martini was adopted as a general frame of reference. However, for age models and inter-site calibration, individual events were used according to their reliability and ease of recognition irrespective of whether they formed part of the standard zonation.
The compilations of Backman et al. For the planktic foraminifers, see Betzler et al. The core to seismic correlation relies on an accurate P wave velocity model for time-depth conversion. Two checkshot surveys were performed at dedicated logging holes UE and UE. At Site U, the waveforms of the recorded shots were of high quality with a high signal-to-noise ratio, and consequently, first arrival times were easy to pick Fig. The seismic and core data set covers the inner part of the Maldivian archipelago Fig.
Two transects of sites aligned along two 2D seismic lines in this area cover the Neogene succession of the Inner Sea of the Maldives Fig. Here, previous work identified a drowned carbonate bank with drift deposits adjacent and overlying the drowned platform Aubert and Droxler , Belopolsky and Droxler a , b ; Betzler et al.
Prior to the drilling of eight sites in the Inner Sea of the Maldives during Expedition , a preliminary sequence analysis had been performed on the seismic data Betzler et al. Erosional truncations and onlap geometries were applied to define sequence boundaries, and 22 sequences were differentiated. Lower-bounding sequence boundaries and overlying correlative seismic sequences were labeled as platform sequence boundaries 1 to 11 PS1—PS11 and platform sequence 1 to 11 ps1—ps11 in the drowned Neogene carbonate bank, and as drift sequence boundary 1 to 10 DS1—DS10 and drift sequences 1 to 10 ds1—ds10 in the drift deposits Figs.
The ages of these sequence boundaries are here examined and refined using the shipboard biostratigraphy. Several of these sequence boundaries delimit stratigraphic packages with distinct growth characteristics. This horizon has been identified as a drowning unconformity terminating the growth of carbonate banks several 10s of kilometers in width, and is the first turning point in the investigated strata.
Shallow water sedimentation eventually resumed on the drowned edifice during the latest Oligocene and Early Miocene. The banks with widths of ca. Furthermore, no reefs can be differentiated in seismic data Aubert and Droxler Subsequently, a shallow-water succession developed in which 11 seismic sequences are recognized Betzler et al. The sequence boundary PS1 is the second turning point and corresponds to the base of a stratigraphic package with a ramp depositional profile, which steepens through time in sequences ps 2, 3, and 4, until a platform with a steep rim forms sequence ps4 Fig.
The same pattern is recognized elsewhere in the Maldives edifice where the seaward progradation is less in the carbonate banks located on the eastern part of the Inner Sea when compared to the western carbonate banks Aubert and Droxler , Progradation is halted in sequence ps5 and changes to a pronounced aggradation in seismic sequences ps6 and ps7. Sequence boundary PS5 marks this transition and is defined as third turning point Fig. At the same time, a backstepping of the outer platform margins facing the open Indian Ocean occurred Betzler et al.
Aggradation changes back into progradation at SB PS8, which is the fourth turning point. Progradation continues in the next three sequences but terminates with sequence ps The end of the progradation marks the most pronounced turning point 5 in the sedimentation pattern, which corresponds to sequence boundary DS1. At this turning point in the upper middle Miocene Fig.
The currents themselves are driven by the monsoon winds that start at These drift bodies are funneled through passages where some platforms had drowned, while remaining banks and atolls continued to grow Betzler et al. These atolls are characterized by aggrading margins Betzler et al. Lobes are fed by easterly currents and reworked by a current system flowing obliquely or normally to this main stream Betzler et al.
The last turning point in the platform evolution, turning point 6 at DS8, is the end of an episode which was characterized by several steps of partial carbonate platform drowning. This process is documented in the studies done by Betzler et al. In the following paragraph, the facies of five different environments that were recovered in the cores are described. They consist of 1 shallow-water carbonate platform, 2 carbonate platform slope, and 3 basin, 4 pelagic chalk, and 5 carbonate drift.