Sediment composition of periplatform sediments and calciturbidites around Pedro Bank, Southwestern Caribbean Sea [Elektronische Ressource] = Sedimentzusammensetzung von Periplatformsedimenten und Kalkturbiditen im Bereich der Pedro Bank, südwestliches Karibisches Meer / vorgelegt von Nis Andresen

Nils Andresen

Informations

• Publié par: christian-albrechts-universitat_zu_kiel
• Publié le: 01 janvier 2000
• Nombre de lectures: 34
• Langue: English
• Poids de l'ouvrage: 32 Mo

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SEDIMENT COMPOSITION OF PERIPLATFORM SEDIMENTS AND
CALCITURBIDITES AROUND PEDRO BANK,
SOUTHWESTERN CARIBBEAN SEA
SEDIMENTZUSAMMENSETZUNG VON PERIPLATFORMSEDIMENTEN UND
KALKTURBIDITEN IM BEREICH DER PEDRO BANK,
SÜDWESTLICHES KARIBISCHES MEER
Dissertation
zur Erlangung des Doktorgrades
der Mathematisch-Naturwissenschaftlichen Fakultät
der Christian-Albrechts-Universität
zu Kiel, Deutschland
vorgelegt von
Nils Andresen
Kiel 2000Referent/ in: Prof. Dr. W.-Chr. Dullo
Korreferent/ in: Prof. Dr. Chr. Betzler
Tag der mündlichen Prüfung: 15. November 2000
Zum Druck genehmigt: Kiel, den 15. November 2000
Der Dekan gez. Prof. Dr. Thomas Bauer
iiABSTRACT
Spatial and temporal variations in the carbonate production and sediment export of the Pedro Bank
carbonate platform were studied in response to the late Quaternary sea level fluctuations. A set of nine
sediment cores were obtained from up- and downcurrent position to Pedro Bank to form a downcurrent
offbank transect that covers the proximal toe-of-slope to a distal basinal setting. The sediments studied
were deposited between 650 to 2520 meter below recent sea level, and reach back until oxygen isotope
stage 9, i.e. until approximately 300-310 ky. The sediments recovered are rather uniform, and consist
of foraminifer- and pteropod-bearing aragonite-rich mud- to wackstones. This typical periplatform
lithology incorporates calciturbidites, which are of pelagic as well as neritic origin. The pelagic sourced
turbidites are generally evident as fine tail-end turbidites, whereas the few neritic calciturbidites are
present as coarse pack- to grainstones.
The sediments were analysed on their ability to record the export patterns of neritic sediment from the
shallow tops of Pedro Bank. Therefore mineralogical, sedimentological and geochemical analyses were
performed. The aragonite and strontium content were found to be good indicators for variations in
shallow-water sediment export. The geochemistry data, as well as the grainsize and component
distribution patterns, were useful to differentiate between primary sediment input signals and secondary
processes such as dissolution and preservation of metastable carbonates at the seafloor.
This study shows that during the last 300 ky (1) sea-level fluctuations, but also the (2) ocean currents,
(3) pelagic and neritic sediment production, (4) dissolution of metastable sediment constituents, and (5)
input of non-carbonates have had a major control on the sediment dispersal, sediment deposition and
composition, especially along the downcurrent margin, where main neritic export occurs. The
mineralogical and grain-size composition documents this variability of periplatform sedimentation along
the margins off Pedro Bank. In addition, it was found that also the seafloor morphology plays an
important role for the spatial and temporal evolution of the periplatform sedimentation in the vicinity of
Pedro Bank, which is situated in a tectonically active plate margin setting.
The analyses of a series of periplatform cores taken from the surrounding of Pedro Bank showed that
based on mineralogical, grain-size and microfacies studies characteristic depositional environments
existed during the last 300 ky, both in space and time. The spatial variability of periplatform
sedimentation shown by the mineralogical and grain-size data along a transect off the leeward margin of
Pedro Bank exhibits the response of the neritic platform environment to climatic and sea-level changes
and its export pattern towards the slopes and into the adjacent basins.
Offbank shedding of neritic sediment components (aragonite and high-Mg calcite) can be observed
within a halo of about 20 km at the downcurrent margin, where main export from the platform occurs.
During interglacial highstands in sea level, the fine sediment fraction (<63 µm) dominates
periplatform sediments along the entire leeward transect and no spatial variations could be observed.
The sediments are dominated by fine aragonite. Within the subclasses of the coarse fraction (>63 µm)
the very fine sand fraction dominates only at proximal sites (<20 km) showing the export potential of
the shallow-water realm. More distal sites (>20 km) exhibit a more bimodal distribution pattern in the
coarse grain sizes with maximum amounts within the very fine and the medium sand fraction,
indication a mixed neritic/pelagic signal. The sediment composition during highstands in sea level
revealed that only minor export of coarse neritic grains into the deep surrounding of Pedro Bank takes
place. The interglacial periplatform sediments are characterised by a low coarse fraction content and a
dominance of coarse planktic grains like planktic foraminifera and pteropods. Microfacies analyses
showed that even in proximal depositional settings close to the toe-of-slope, coarse bank-derived grains
only occurred at an average of about 4 vol.%. The individual groups of coarse shallow-water grains do
not show any clear glacial/interglacial distribution pattern, whereas planktic foraminifera and
pteropods, which form major parts of the coarse fraction, show very similar temporal variations with
high amounts during glacials and low amounts during interglacial periods.
During glacial lowstands in sea level a twofold division is evident in the spatial distribution of the
periplatform sediments. A proximal environment (<28 km) with enhanced percentages in coarse
fraction content (22-33%) vs. a distal environment (> 28 km) with a strong dominance of the fine
fraction (> 90%). The increased coarse fraction content at the proximal sites results from various
interacting contemporaneous processes: (1) lower input of fine neritic sediments, (2) increased current
winnowing, and (3) redepositional processes at the upper slope during lowered sea-level stands, and
the export of this material to “proximal basinal” sites (< 28 km). In the coarse fraction subclasses a
similar distribution pattern is found at all sites showing a bimodal distribution. This might show the
low neritic influence, and a similar influence of pelagic sedimentation at all sites during glacials. During
iiiglacials, when preservation of metastable carbonate minerals is enhanced at intermediate water depths,
aragonite still shows a general decrease with greater offbank distance, whereas HMC shows highest
accumulation rates at about 1000-1400 mbsl (= 20-42 km offbank distance), which might result from
submarine precipitation of HMC-cements or the erosion and redeposition of these cements from upper
slope settings combined with better preservation potential under the influence of intermediate water
masses.
The mineralogy, turbidite distribution as well as composition analyses of the periplatform sediments
deposited during lowered interstadial sea-level stand of marine isotope stage 3 might be
indicative for enhanced neritic production at the top of Pedro Bank. This productivity might be caused
by the inclined bank top morphology with its deepest parts at the northwestern edge.
In upcurrent locations the carbonate mineralogy displays a similar spatial evolution as seen along the
downcurrent margin, but with overall reduced amounts. This is due to the lower export potential of the
platform regime against the main direction of the Caribbean Current, one important factor for neritic
sediment dispersal in the study area. Turbidites and coarse glacial periplatform sediments from
upcurrent settings show a slightly higher abundance of typical shallow-water grains, which mirrors the
different faunal composition at the top of Pedro Bank with its shallower bank top depths in comparison
to the downcurrent margins.
18Lagtimes between sea-level and the timing of deposition of sediment export proxies (e.g. δ O vs.
aragonite) were observed within several sediment cores from the surrounding of Pedro Bank. These
lagtimes probably were caused by the inclined bank-top morphology, which results in a different
timing of bank-top flooding of Pedro Bank on either the northern or southern margin. This pattern
could not be observed in all cores, which might be the result from a variety of other processes such as:
(1) early diagenesis, (2) relative position to the bank margin, (3) seafloor morphology and (4) syn- and
postdepositional dissolution of metastable carbonates, which might might have affected the metastable
sediment constituents to a varying degree.
The study of calciturbidites shows that the highstand “bundling” of calciturbidites is well developed
even in the tectonically active setting of the Northern Nicaragua Rise close to the Caribbean plate
boundary. The high- to lowstand turbidite frequency for the last 300 ky is the same with 3 times more
highstand than lowstand turbidites during the last 3 glacial-interglacial cycles. This consistency might
indicate that no major depositional changes occurred within the Pedro Bank carbonate system during
this time interval. Turbidite deposition in the vicinity of Pedro Bank is controlled by: (1) late
Quaternary sea-level fluctuations, (2) sediment production and overloading of the upper slope, (3)
proximity of the core location to the bank margin, (4) local slope morphology and (5) seafloor
morphology, which provides a line source for turbidites through gullies and canyons. At the end of
these canyon and gully systems the turbid