531 |
Sedimentology, coral reef zonation, and late Pleistocene coastline models of the Sodwana Bay continental shelf, Northern ZululandRamsay, Peter John. January 1991 (has links)
This geostrophic current-controlled Zululand/Natal shelf displays a unique assemblage of interesting
physical, sedimentological and biological phenomena. The shelf in this area is extremely narrow
compared to the global average of 75km, and is characterised by submarine canyons, coral reefs, and
steep gradients on the continental slope. A shelf break occurs 2.1km to 4.1km offshore and the shelf
can be divided into a northern region and a southern region based on the presence or absence of a
defined shelf break. The southern shelf has a poorly-defined shelf break whilst the northern shelf has
a well-defined break at -65m. The poor definition of the shelf break on the southern shelf can possibly
be attributed to the presence of giant, climbing sand dunes offshore of Jesser Point at depths of -37m
to -60m. The northern shelf has a series of coast-parallel oriented patch coral reefs which have
colonised carbonate-cemented, coastal-facies sequences.
The northern shelf can be divided into three distinct zones: inner-, mid-, and outer-shelf zones. The
inner-shelf is defined as the area landward of the general coral reef trend, with depths varying from
0m to -I5m and having an average gradient of 1.1. The mid-shelf is defined by the general coral reef
trend, varying from -9m over the shallow central axis of the reefs to -35m along the deep reef-front
environments. The outer-shelf is seaward of the coral reefs and occurs at a depth range of -35m to -
65m. Gradients vary from 1° in the south to 2.5° in the northern part of the study area, and are steep
compared to world average shelf gradient of 0.116°.
Four submarine canyons occur in the study area and are classified as mature- or youthful-phase
canyons depending on the degree to which they breach the shelf. The origin of these canyons is not
related to the position of modern river mouths but can probably be linked to palaeo-outlets of the
Pongola and Mkuze River systems. It is suggested that the canyons are mass-wasting features which
were exploited by palaeo-drainage during regressions. The youthful-phase canyons appear to be mass-wasting features associated with an unstable, rapidly-deposited, progradational late Pliocene sequence
and a steep upper continental slope. The mature-phase canyons were probably initiated by mass-wasting
but have advanced shoreward, breaching the shelf, due to their link with the palaeo-outlets
of the Pongola and Mkuze Rivers during late Pleistocene regressions.
Evidence of modem canyon growth has been noted on numerous SCUBA diving surveys carried out
on the canyon heads. These take the form of minor wall slumps and small-scale debris flows. The
canyons are also supplied with large quantities of sand in the form of large-scale shelf subaqueous
dunes generated and transported by the Agulhas Current. As these bedforms meet the canyons the
sediment cascades down the canyon thalweg and causes erosion and downcutting of the canyon walls
and floor thereby increasing the canyon dimensions.
Late Pleistocene beachrock and aeolianite outcrops with or without an Indo-Pacific coral reef veneer
are the dominant consolidated lithology on the shelf. These submerged, coast-parallel, carbonate cemented,
coastal facies extend semi-continuously from -5m to -95m, and delineate late Pleistocene
palaeocoastline events. The rock fabric of these high primary porosity lithologies shows grains floating
in a carbonate cement with occasional point-contacts. Grains are mostly quartz (80-90%), minor K-feldspar
and plagioclase (5-10%), and various lithic fragments. The rocks contain conspicuous organic
grains including foraminifera, bivalve, echinoid, bryozoan, red algal, and occasional sponge spicule
fragments; these commonly display replacement fabrics or iron-stained rims. The dominant
sedimentary structures found in these sandstone outcrops include high-angle planar cross-bedding and
primary depositional dip bedding. Palaeocurrent directions sngest a palaeoenvironment dominated
by a combination of longitudinal and transverse dunes with wind directions similar to those observed
forming the modem dune systems. Erosional features evident on the submerged beachrocks and
aeolianites include gullies trending in two different directions and sea-level planation surfaces with or
without the presence of potholes.
The unconsolidated sediment on the shelf is either shelf sand, composed mainly of terrigenous quartz
grains; or bioclastic sediment which is partially derived from biogenic sources.
The quartzose sand from the inner-shelf is generally fine-grained, moderately- to well-sorted, and
coarsely- to near symmetrically-skewed. Carbonate content is low, and varies between 4-13%.
Quartzose sand from the outer-shelf is fine-grained, moderately- to well-sorted, and coarsely- to very
coarsely-skewed. The inner-shelf quartzose sand is better sorted than the outer-shelf sand due to
increased reworking of this sediment by the high-energy swell regime. Sediment from the shallower
areas of the outer-shelf (< -50m) is better sorted than sediment from depths of greater than -50m.
Generally wave-reworking of quartzose shelf sand from the Sodwana Bay shelf results in greater
sediment maturity than that observed from geostrophic current effects or a combination of geostrophic
and wave-reworking. This sediment was derived by reworking of aeolian and beach sediments,
deposited on the shelf during the period leading up to the Last Glacial Maximum (15 000 - 18 000
years B.P.) when sea-level was -130m, during the Holocene (Flandrian) transgression.
Bioclastic sediment on the Sodwana Bay shelf is defined as having a CaC03 content of greater than
20% and is a mixture of biogeoically-derived debris and quartzose sand. The distribution of bioclastic
sediment in the study area is widespread, with reef-derived and outer-shelf-derived populations being
evident. This sediment consists of skeletal detritus originating from the mechanical and biological
destruction of carbonate-secreting organisms such as molluscs, foraminifera, alcyonaria, scleractinia,
cirripedia, echinodermata, bryozoa, porifera. The reef-derived bioclastic population is confined to
depths less than -40m in close proximity to reef areas, whereas the shelf-derived bioclastic population
occurs at depths greater than -40m and is derived from carbonate-producing organisms on deep water
reefs and soft-substrate environments on the shelf.
Large-scale subaqueous dunes form in the unconsolidated sediment on the outer-shelf due to the
Agulhas flow acting as a sediment conveyor. These dunes are a common feature on the Sodwana Bay
shelf occurring as two distinct fields at depths of -35m to -70m, the major sediment transport direction
being towards the south. The two dune fields, the inner- and outer subaqueous dune fields, are
physically divided by Late Pleistocene beachrock and aeolianites ledges. A bedform hierarchy has been
recognised. The larger, outer dune field appears to have originated as a system of climbing bedforms
with three generations of bedforms being superimposed to form a giant bedform, while the inner dune
field has a less complex construction. The largest bedforms are those of the outer dune field off Jesser
Point, being up to 12 m high, 4 km long and 1.2 km wide. A major slip face, with a slope of 8° is
present.
Bedload parting zones exist where the bedform migration direction changes from south to north.
Three bedload parting zones occur in the study area at depths of -60m, -47m and -45m; two in the
inner dune field and one in the outer dune field. These zones are invariably located at the southern
limits of large clockwise eddy systems. Such eddies appear to be the result of topographically induced
vorticity changes in the geostrophic flow and/or the response to atmospheric forcing caused by coastal
low-pressure system moving up the coastline.
It has been demonstrated that the inner subaqueous dune sediment conveyor is not active all the time
but only during periods . of increased current strength when the Agulhas Current meanders inshore.
The smaller bedforms in the outer dune field undergo continuous transport due to the current velocity
on the shelf edge outer dune field being higher than the velocity experienced on the inner dune field.
The very large 2·D dune which forms the outer dune field is probably not active at present: this is
inferred due to the shallow angle of the mega-crest lee slope (8°).
The very large Sodwana Bay subaqueous dune fields may be compared with the very large,
reconstructed, subaqueous dunes which occur in Lower Permian sediments of the Vryheid Formation,
northern Natal. These Permian dunes are represented, in section, as a fine- to medium-grained distal
facies sandstone with giant crossbeds. These large-scale bedforms are unidirectional, but rare
directionally-reversed, climbing bedforms do occur, this directional reversal may be related to bedload
parting zones. On the evidence presented in this thesis, it is proposed that these Permian subaqueous
dunes may be ancient analogues of the modem subaqueous dune field on the Sodwana Bay shelf.
Positive-relief hummocks and negative-relief swale structures are fairly common in the fine-grained,
quartzose shelf sand at depths of -30m to -60m. These appear to be transitional bedforms related to
the reworking by storms of medium 2-D subaqueous dunes. These hummocky structures may be the
modem equivalent of hummocky cross-stratification noted in the geological record, and if so, they are
probably the first to have ever been observed underwater.
The occurrences of ladderback ripples on the Sodwana Bay shelf at depths of -4m to -17m, suggest that
subtidal ladderback ripples may be more common than previously thought. Ladderback ripples are
common features of tidal flats and beaches where they form by late-stage emergence run-off during
the ebb tide. They are generally considered diagnostic of clastic intertidal environments. The mode
of formation on the Sodwana Bay shelf is different from the classic late-stage emergence run-off model
of intertidal occurrences, being a subtidal setting. Subaqueous observations indicate that ladderback
ripples are not environment-specific, and that additional evidence of emergence is therefore necessary
to support an intertidal setting in the rock record: ladderback ripples alone are insufficient to prove
an intertidal environment.
The coral patch reefs of the northern Natal coast are unique, being the most southerly reefs in Africa,
and totally unspoilt. The Zululand reefs are formed by a thin veneer of Indo-Pacific type corals which
have colonised submerged, late Pleistocene beachrocks and aeolianites. Two-Mile Reef at Sodwana Bay
has been used to develop a physiograpbic and biological zoning model for Zululand coral reefs, which
has been applied to other reefs in the region. Eight distinct zones can be recognised and differentiated
on the basis of physiographic and biological characteristics. The reef fauna is dominated by an
abundance of alcyonarian (soft) corals, which constitute 60-70% of the total coral fauna. The Two-Mile
Reef zoning model has been successfully applied to larger reefs such as Red Sands Reef, and smaller
patch reefs (Four-Mile and Seven-Mile Reefs) in the same general area.
In this thesis extensive use has been made of Hutton's uniformitarian principles. Hutton's doctrine
is particularly relevant to the study of depositional processes and relict shorelines. Coastal processes
and weather patterns during the late Pleistocene were broadly similar to modem conditions enabling
direct comparisons to be made. A computer-aided facies analysis model has been developed based on
textural statistics and compositional features of carbonate-cemented coastal sandstones. Many
attempts have been made to distinguish different ancient sedimentary depositional environments, most
workers in this field having little success. The new method of facies reconstruction is based on:
(1) underwater observations of sedimentary structures and general reef morphology;
(2) a petrographic study of the reef-base enabling flve facies: aeolianite, backbeach,
forebeach, swash, and welded bar facies to be recognised, which control the
geomorphology of Two-Mile Reef;
(3) cluster and discriminant analysis comparing graphic settling statistics of acid-leached
reef-base samples with those of modem unconsolidated dune/beach environments.
The results of this analysis demonstrated that the beachrocks and aeolianites on the shelf formed
during a regression and that late Pleistocene coastal facies are similar to modem northern Zululand
coastal environments, which have been differentiated into aeolian, backbeach, forebeach, swash, &
welded bar.
A late Pleistocene and Holocene history of the shelf shows that during the late Pleistocene, post
Eemian regressions resulted in deposition and cementation of coast-parallel beachrocks and
aeolianites, which define a series of four distinct palaeocoastline episodes with possible ages between
117 000 and 22 000 years B.P. The beachrock/aeolianites formed on the shelf during stillstands and
slow regressions, and the gaps between these strandline episodes represent periods of accelerated sealevel
regression or a minor transgressive phase which hindered deposition and cementation. The
formation of these lithologies generated a considerable sediment sink in the nearshore zone. This
reduced sediment supply and grain transport in the littoral zone during the Holocene, and probably
enhanced landward movement of the shoreline during the Flandrian transgression.
Prior to the Last Glacial Maximum, the beachrock/aeolianite sedimentary sequence was emergent and
blanketed by shifting aeolian sands. The Pongola River, which flowed into Lake Sibaya, reworked the
unconsolidated sediments on the shelf, and exploited the route of least resistance: along White Sands
and Wright Canyon axes. The erosion resulting from fluvial denudation in Wright Canyon has caused
this canyon to erode some of the beachrock/aeolianite outcrops which form palaeocoastline episode 2
and entrench the canyon to a deeper level; this eroded the shelf to a distance of 2km offshore.
During the Flandrian transgression the unconsolidated sediment cover was eroded, exposing and
submerging the beachrock/aeolianite sequence. Flandrian stillstands caused erosional features such
as wave-planed terraces, potholes, and gullies to be incised into beachrock and aeolianite outcrops;
these are seen at present depths of -47m, -32m, .26m, -22m, -17m to -15m, and -12m. High energy
sediment transfers, in an onshore direction, resulted in the deposition of sand bars across the outlet
of Lake Slbaya's estuary and the development of a 130m + coastal dune barrier on a pre-existlng,
remnant Plelstocene dune stub. Sea-level stabilised at its present level 7 000-6 000 years B.P. and coral
reef growth on the beachrock/aeolianite outcrops probably started at 5 000 years B.P. A minimum age
for the formation of the northern Zululand coral reefs has been established at 3780 ± 60 years B.P.
A mid Holocene transgression relating to the Climatic Optimum deposited a + 2m raised beach rock
sequence. This transgression eroded the coastal dune barrier and caused a landward shoreline
translation of approximately 40m. A minor transgression such as this can be used as a model for
coastal erosion which will result from the predicted 1.5m rise in sea-level over the next century. This
rise in sea-level could result in a 30m landward coastline translation of the present coastline, ignoring
the influence that storms and cyclones will have on the coastline configuration. / Thesis (Ph.D.)-University of Natal, Durban, 1991.
|
532 |
Determining Hillslope Diffusion Rates in a Boreal Forest: Quaternary Fluvial Terraces in the Nenana River Valley, Central Alaska RangeWalker, Laurel Anne 01 January 2014 (has links)
The subarctic boreal forest biome is predicted to experience higher magnitudes of warming than other biomes due to climate change. The effects of this warming will be pronounced in areas underlain by discontinuous permafrost where melting permafrost and distinct changes in vegetation patterns are expected. To better understand rates of hillslope diffusion in the boreal forest I have used a geomorphic process modeling approach, using data from a sequence of Quaternary fluvial terraces located in the Nenana River valley of central Alaska. I hypothesized that diffusion rates here would be slower when compared to the mid-latitudes, and faster on north versus south-facing slopes. Calculated diffusion rates do support the hypothesis as they fall on the lower end of the global spectrum of documented hillslope diffusion rates. However, a significant difference in diffusion rates is not seen between the predominantly northeast and southwest facing slopes used in this study.
|
533 |
Spatio-temporal variations of the sedimentology and geochemistry of six estuaries within the eThekwini Municipality, KwaZulu-Natal, South Africa.Pather, Keshia. 15 September 2014 (has links)
Estuaries are dynamic features of a coastline whose sediments are influenced by riverine and
marine processes. Periodic events such as floods, as well as variations in mouth status,
greatly affect the energy levels within an estuary and subsequently the amount of sediment
erosion and deposition that takes place. Concurrently, pollutants are transported and
deposited into estuaries and can reside in the sediments for many years. The estuaries of the
eThekwini Municipality in KwaZulu-Natal, South Africa, are exposed to a variety of
pollutants; however with the expanding industrial sector within this region, metal
contamination is of concern.
This study investigates the sedimentology and geochemical variations of six estuaries within
the municipality namely, the uTongati, uMdloti, uMgeni, Isipingo and uMbokodweni
estuaries as well as the Durban Harbour. To determine the spatial variations in estuarine
sedimentology, sediment cores were collected longitudinal to the estuary axis. The core
samples were analysed for sediment colour, texture and organic matter content. To assess the
geochemical variations, core samples were analysed for Zn, Cu, Cr, Ni, Pb, As, Fe, Al, Ca, S,
P, Mg, Mn, Cd and V concentrations. Some samples were also carbon dated to provide a
temporal aspect to the sediment and geochemical variations. Descriptive and graphic
techniques were used to examine the sedimentology within the estuaries; and the geochemical
data was analysed with the use of multivariate statistics. Additionally, pollution indices and
sediment quality guidelines were utilized to assess the pollution levels within the sediments.
The results indicated that lower energy environments caused by protracted mouth closures in
the uMdloti and Isipingo estuaries accounted for large amassing of fines. In contrast, the
accumulation of mixed coarse and fine sediments in the uTongati and uMgeni estuaries was
an indication of high fluvial flows and open mouth conditions. All carbon dated ages for all
estuaries were greater than 700 years which may be attributed to a combination of scouring
effects from past and recent flood events and also possibly due to the deposition of re-worked
older sediments from upstream. Low metal concentrations were found within the sediments
of all estuaries, and the presence of fines and organic matter governed their concentration
variations with depth. The uMgeni and uMbokodweni estuaries which are located
immediately downstream of industrial and urban areas, were found to contain relatively
higher concentrations of elements Pb, Cu, As and Ni. These metals showed high enrichment
within the sediments; however actual concentrations were below sediment quality guideline
levels. General pollution levels within all estuaries were very low, and can be attributed to the climatic influences within this region which has a ‘cleansing’ effect on the estuarine
environments in removing contaminants. / Thesis (M.Sc.)-University of KwaZulu-Natal, Durban, 2014.
|
534 |
Sedimentology, stratigraphy, and provenance of the upper Purcell Supergroup, southeastern British Columbia, Canada: implications for syn-depositional tectonism, basin models, and paleogeographic reconstructionsGardner, David William 29 April 2008 (has links)
This thesis reports eight measured sections and >400 new detrital zircon U-Pb SHRIMP-II ages from the Mesoproterozoic (~1.4 Ga) upper Purcell Supergroup of southeastern British Columbia, Canada. The goal of my study is to constrain the depositional, tectonic and paleogeographic setting of the upper Purcell Supergroup.
Stratigraphic sections across the Purcell Anticlinorium, constructed from measured sections, reveal three syn-depositional growth faults: (1) paleo-Hall Lake, (2) paleo-Larchwood Lake, and (3) paleo-Moyie. Stratigraphic sections were combined into a fence diagram, revealing a large north-northeast trending graben bound to the east by the paleo-Larchwood Lake fault and to the west by the paleo-Hall Lake fault.
Five samples were collected for detrital zircon analysis along the eastern extent of exposed Purcell strata; one sample was collected from the western limit of strata. All samples are characterized by subordinate numbers of detrital zircons that yield Paleoproterozoic and Archean ages. Detrital zircon ages from the Sheppard Formation are dominated by 1500, 1700, 1750, and 1850 Ma grains. The overlying Gateway Formation is dominated by 1400-1450, 1700, 1850, and 1900 Ma zircon grains. The overlying Phillips, Roosville (east), and Mount Nelson formations are dominated by detrital zircon ages between 1375-1450 Ma and 1650-1800 Ma. Detrital zircon ages from the Roosville Formation (west) are dominated by 1500-1625 Ma grains.
Based on the margin perpendicular orientation of the long axis of syn-depositional grabens relative to Laurentia, and on the presence of syn-depositional aged zircons through the entire sedimentary succession, we interpret the upper Purcell Supergroup to have been deposited in a transpressional pull-apart basin setting, adjacent to a convergent/translational plate margin bound to the west by terranes now located in northeastern Australia.
|
535 |
Eco-sedimentological environments of an inter-tidal reef platform, Warraber Island, Torres StraitHart, Deirdre E., Physical, Environmental & Mathematical Sciences, Australian Defence Force Academy, UNSW January 2003 (has links)
This thesis examines functional relationships between the morphologic, hydrodynamic, ecological and sedimentological characteristics of the Warraber reef platform, an inter-tidal reef island system, Central Torres Strait, Australia (10[degrees] 12??? S, 142 [degrees] 49??? E). Hydrodynamic and sediment-transport experiments were conducted on the reef flat using current meters, water level recorders and directional sediment traps. Results showed dominantly SE flows during the dry season and more variable NW to SE flows during the wet season. Topography and reefal water levels modulated the direction and strength of currents and the generation of wind-waves on the reef flat as well as the passage of waves over the reef rim. These hydrodynamic conditions are sufficient to induce significant transport of moderately fast to slow settling sediment (>-5.25 symbol psi) on the reef flat, though the platform as a whole is a relatively closed transport system. Carbonate production was estimated based on the key ecological variables of live assemblage distribution and cover. Overall, only 24% of the reef flat was occupied by carbonate-producing organisms. The average estimated carbonate-production rate for the reef was 1.6 kgm -2y-1 (0.07-4.37 kgm-2y-1). Production is dominated by coral (73%), with subordinate proportions contributed by coralline algae (19%). And molluscs, foraminifera and Halimeda (<4%) though actual reef-flat sediments did not reflect this potential. Instead, they were dominated by molluscs (35-55%), coralline algae (16-26%), coral (8-13%), Halimeda (7-8%) and foraminifera (5-10%). Differential rates of carbonate to sediment conversion meant the reef-platform sediments were more closely related to the cover of live organisms than to the contribution of carbonate production by each parent organism. The settling properties of the least altered particles of the five commonest constituents were measured and these provided the basis for an eco-sedimentological model of the reef-platform system. Modelled textures were compared to the actual textures, indicating the degree of textural alteration resulting from a combination of biological and physical processes, including sediment production, hydraulic sorting and mechanical breakdown. This analysis, integrated with the hydrodynamic, exposure and other data, was used to determine reef-platform surface-sediment sources, sinks and transport pathways. In using both the textual and constituent compositional properties of sediments, as well as information on local biological and physical processes, the model approach developed offers progress towards an integrative, interdisciplinary analysis of carbonate environments.
|
536 |
Impact des variations eustatiques sur les assemblages benthiques à Brachiopodes : l'ordovicien Sarde et le Dévonien Ibéro-ArmoricainBOTQUELEN, Arnaud 17 December 2003 (has links) (PDF)
Ce travail de thèse avait un double objectif: préciser les relations entre les organismes marins paléozoïques et les changements paléoenvironnementaux et si possible établir un modèle applicable dans différents domaines paléogéographiques et à différentes époques du Paléozoïque. Dans ce but, des analyses sédimentologiques, paléoécologiques et taphonomiques ont été réalisées afin de déterminer la réponse de la faune benthique aux variations du niveau marin, dans un contexte environnemental de plate-forme, à 2 époques différentes, l'Ordovicien supérieur et le Dévonien inférieur. Une coupe du Caradoc-Ashgill de Sardaigne, cinq coupes de l'Emsien inférieur du Massif armoricain et une coupe de l'Emsien inférieur d'Espagne (León) ont été choisies. La démarche de l'étude consiste en une analyse séquentielle à haute résolution et en un échantillonnage détaillé, destiné à l'étude paléontologique (taphonomie, définition d'associations d'organismes récurrents, systématique). Dans ce travail, seuls les brachiopodes ont été étudiés du point de vue systématique. Les brachiopodes décrits et figurés comportent 20 espèces pour l'Ordovicien supérieur sarde et 33 espèces pour le Dévonien inférieur Ibéro-armoricain. L'étude stratonomique a permis de reconnaître plusieurs types de faciès sédimentaires et les conditions de dépôt, en relation avec la distance à la ligne de rivage. L'empilement des faciès sédimentaires s'organise en séquences génétiques engendrées par les fluctuations du niveau relatif de la mer. Le signal de variation du niveau marin est cyclique et semble lié aux changements des paramètres orbitaux de Milankovitch. Des associations d'organismes, définies à l'aide d'analyses multivariées (analyses en composantes principales, classification hiérarchique), se répartissent de façon récurrente et en accord avec les faciès sédimentaires, caractérisant ainsi des milieux de dépôt différents. 16 associations sont ainsi définies: l'association à 'Longvillia', 'Nicolella', 'Paucicrura' et 'Ibéromena' de l'Ordovicien supérieur de Sardaigne; l'association à 'Adolfia', 'Stenorhynchia', 'Chonétides', 'Crinoïdes-Brachiopodes', 'Crinoïdes-Ostracodes', 'Ostracodes', 'Ostracodes-Tentaculitides', 'Bryozoaires' et 'Dacryoconarides' du Dévonien inférieur du Massif armoricain; l'association à 'Rhynchonellides', 'Spiriférides' et 'Tétracoralliaires-Crinoïdes-Dacryoconarides' du Dévonien inférieur ibérique. Une grande partie des archives paléontologiques est conservée au sein de concentrations coquillières d'épaisseur variable. L'étude des concentrations coquillières, en considérant les différents critères taphonomiques et leur répartition au sein des séquences génétiques, a permis la distinction de 2 types de concentration et l'élaboration d'un modèle expliquant leur mise en place. La formation des concentrations coquillières est liée à la dynamique des tempêtes (facteurs autocycliques) et aux variations du niveau marin (facteurs allocycliques). Les concentrations fines de tempêtes (type A) se forment préférentiellement lors des phases de chute du niveau marin à très haute fréquence, tandis que les concentrations composites de tempêtes (type B) s'individualisent lors des périodes d'élévation du niveau de la mer à très haute fréquence. Ces périodes se caractérisent par des apports sédimentaires faibles: il s'agit d'intervalles de stabilité à l'interface eau-sédiment, favorisant la rétroaction taphonomique ou encore l'activité des organismes perforants (bioérosion). Les accumulations coquillières qui en résultent sont des niveaux condensés, caractérisées par l'amalgame de nombreuses générations d'organismes. La complémentarité des approches séquentielles et paléontologiques se révèle fructueuse dans la compréhension de la dynamique des associations: le résultat des analyses multivariées des niveaux à partir de leur contenu faunique est en accord avec les regroupements établis par la stratigraphie séquentielle. Les associations reflètent clairement les fluctuations du niveau marin. Lors des variations progressives et modérées du niveau marin, les remplacements d'associations se caractérisent par une réorganisation de la composition taxonomique, de la diversité et des réseaux trophiques (remplacements d'associations par réorganisation). Lors de variations rapides et importantes du niveau marin, les remplacements d'association se définissent par des renouvellements dans la structure écologique (remplacements d'associations par renouvellement). Des changements paléoécologiques sont ainsi définis en relation avec les variations eustatiques.
|
537 |
Mineralogia de solos e ambientes de sedimentação em manguezais do Estado de São Paulo / Soil mineralogy and sedimentary environment in mangroves of São Paulo StateValdomiro Severino de Souza Júnior 22 February 2006 (has links)
Manguezais são formados por grupos de árvores e arbustos que se desenvolvem na zona de intermarés de regiões tropicais. Este ecossistema ao se estabelecer na interface do ambiente marinho e continental, apresenta sua formação relacionada com as flutuações relativas do nível do mar quaternário, através do preenchimento dos vales dos rios, margens de lagunas e baías com sedimentos tanto de origem continental como marinha. O conhecimento da distribuição de partículas e dos minerais constituintes da assembléia mineralógica dos solos em ambiente de planícies estuarinas, auxilia na compreensão de processos de sedimentação e geoquímica dentro dos estuários. O trabalho foi realizado nos manguezais distribuídos ao longo do litoral do Estado de São Paulo, e objetivou-se caracterizar os ambientes de sedimentação de acordo com a granulometria e o processo de evolução quaternária, determinar qualitativamente e semiquantitativamente a assembléia mineralógica e estudar em detalhe os tipos de esmectitas presentes nesses solos. Para tanto foram coletadas amostras de solos de 14 manguezais nas camadas de 0-20 e 60-80cm e de sedimento em suspensão do Rio Ribeira de Iguape, além de amostras de diferentes profundidades para datações. Determinaram-se as frações, argila, silte, areia total e 5 frações da areia, foram realizadas datações 14C por cintilação líquida e AMS na fração humina da matéria orgânica e por termoluminescência em grãos de quartzo. As análises mineralógicas foram realizadas na fração silte e argila e no sedimento em suspensão através de DRX, IV, MET, MEV com microanálise, ATD e ATG, realizou-se também o teste de Greene- Kelly para identificar o tipo de esmectitas presentes nesses solos. De acordo com os resultados obtidos, concluiu-se que os solos dos manguezais do Estado de São Paulo podem apresentar diferentes texturas desde arenosa até muito argilosa, freqüentemente contêm altos teores de silte e todos têm idade holocênica, oscilando entre 410 anos A.P. e 3.700 anos; a distribuição de partículas está relacionada à morfologia atual do estuário e à natureza dos sedimentos costeiros adjacentes; a assembléia mineralógica da fração fina dos solos é constituída de pirita, nontronita, caulinita, illita, gibbsita, quartzo, feldspato, e localmente ocorrem goethita, vermiculita, biotita, halloysita e anatásio; os minerais alóctones são de origem continental e marinha; a distinção entre os cenários geomorfológicos ao longo do litoral condiciona a distribuição de minerais, e ainda foi inferido que ocorre neoformação de esmectita e de caulinita, e que o processo de oxidação de matéria orgânica nesses solos pode estar utilizando o Fe3+ da nontronita como receptor de elétrons. / Mangroves are formed by groups of trees and shrubs that develop in the intertidal zone of tropical regions. This ecosystem to establish in the interface of both marine and continental environment, present its formation related to the sea-level fluctuations during the quaternary period, where the terrigenous and marine sediments are deposited in rivers valleys, edges of lagoons and bays. The knowledge of both particle distribution and minerals constituent of soils in estuarines areas, can aid to understand the processes of sedimentation and geochemistry in this site. This study was carried out with mangroves distributed along the São Paulo State coast, and aims to characterize the sedimentary environments in accordance with grain size and process of quaternary evolution, to determine qualitatively and semiquantitatively the mineralogical assemblage and to identify the smectites types in these soils. Samples were collected from 14 mangroves soils at the layers 0-20 and 60-80cm, and also was collected sediment in suspension of the Ribeira de Iguape River, and samples of different depths for dating. The clay, silt and total sand sizes and 5 sand fractions were determined, radiocarbon dating were carried out by liquid scintillation counting and accelerator mass spectrometry in humin fraction of the soil organic matter and by thermoluminescence of quartz grains. The mineralogical analyses were carried out by XRD, FTIR, TEM, SEM, DTA and GTA in silt and clay sizes and in the sediment in suspension, also was made the Greene-Kelly test to identify the smectites type. According to the results we concluded that the mangroves soils from São Paulo State have different textures varying from sandy up to very clay, also occuring high silt contents. All the mangroves are holocenic, with ages varying from 410 yr B.P. to 3,700 yr; the particle distribution is related to the current geomorphological setting of the estuary and the origin of coastal sediments. The mineralogical assemblage is constituted of pyrite, nontronite, kaolinite, illite, biotite, gibbsite, quartz, feldspars, and locally occurs goethite, vermiculite, halloysite and anatase; the aloctones minerals are from both the terrigenous and marine origin; the difference between geomorphological settings along the coastal plain rules mineral distribution, and still it was inferred the neoformation of esmectita and kaolinite and that the process of organic matter mineralization in these soils may be using Fe3+ from nontronite as an electron acceptor.
|
538 |
Modelagem sedimentar de sistemas deltaicos altamente eficientes: aplicação para o preenchimento sedimentar do terciário superior da bacia de Bengala, porção nordeste da Índia / High efficient deltaic system modeling: application for upper tertiary basin fill history of Bengal basin, Northeast IndiaMarlon Santos Delai 15 September 2011 (has links)
A bacia de Bengala, localizada a Nordeste da Índia tem uma história evolutiva extraordinária, diretamente controlada bela fragmentação do Gondwana. O início da formação desta bacia é considerada como sendo relacionada ao final do evento da quebra, datado em 126 Ma quando a Índia separou do continente Antártico e da Austrália. Desde então, a placa continental Indiana viajou do pólo sul a uma velocidade muito rápida (16 cm/a) chocando-se com o hemisfério norte e fundindo-se com a Placa Eurasiana. Durante a viagem passou por cima de um hot spot, onde hoje estão localizadas as ilhas Seicheles, resultando em um dos maiores derrames de lava basáltica do mundo, conhecido como Deccan Trap. Na região onde a bacia de Bengala foi formada, não houve aporte significativo de sedimentos siliciclásticos, resultando na deposição de uma espessa plataforma carbonática do Cretáceo tardio ao Eoceno. Após este período, devido a colisão com algumas microplacas e a amalgamação com a Placa Eurasiana, um grande volume sedimentar siliciclástico foi introduzido para a bacia, associado também ao soerguimento da cadeia de montanhas dos Himalaias. Atualmente, a Bacia de Bengala possui mais de 25 km de sedimentos, coletados neste depocentro principal. Nesta dissertação foram aplicados conceitos básicos de sismoestratigrafia na interpretação de algumas linhas regionais. As linhas sísmicas utilizadas foram adquiridas recentemente por programa sísmico especial, o qual permitiu o imageamento sísmico a mais de 35km dentro da litosfera (crosta continental e transicional). O dado permitiu interpretar eventos tectônicos, como a presença dos Seawards Dipping Reflectors (SDR) na crosta transicional, coberto por sedimentos da Bacia de Bengala. Além da interpretação sísmica amarrada a alguns poços de controle, o programa de modelagem sedimentar Beicip Franlab Dionisos, foi utilizado para modelar a história de preenchimento da bacia para um período de 5,2 Ma. O nível relativo do nível do mar e a taxa de aporte sedimentar foram os pontos chaves considerados no modelo.
Através da utilização dos dados sísmicos, foi possível reconhecer dez quebras de plataformas principais, as quais foram utilizadas no modelo, amarrados aos seus respectivos tempos geológico, provenientes dos dados dos poços do Plioceno ao Holoceno. O resultado do modelo mostrou que a primeira metade modelada pode ser considerada como um sistema deposicional retrogradacional, com algum picos transgressivos. Este sistema muda drasticamente para um sistema progradacional, o qual atuou até o Holoceno. A seção modelada também mostra que no período considerado o total de volume depositado foi em torno de 2,1 x 106 km3, equivalente a 9,41 x 1014 km3/Ma. / The Bengal Basin in Northeast India has a remarkable evolution history directly controled by the Gondwana fragmentation. The Beginning of this basin is considered related to the final break up event, dated as 126 Ma when India finally became separeted from Antarctica and Australia Continents. Since then, the India Continental Plate traveled from the South pole at very fast rate (16 cm/y) reaching the nortern hemisphere and merging with Eurasia Plate. During the travel it passed over a hot spot, located where is today Seychelles Island, resulting in one of the biggest basalt leakage, know as Deccan Trap. In the area where the Bengal Basin was formed during the travel there wasnt any supply of siliciclastics, resulting in the deposition of a very thick section of carbonate platforms from Late Cretaceous to Eocene. After this period, due to the collision with same microplates and the amalgamation with Eurasia Plate a huge siliciclastic supply was introduced in the basin, associated with rising of the Himalayas Mountains. Therefore, the Belgal Basin today has more than 25 km of sediments, collected in its main depocenter. In this dissertation it was applied to the basic concepts of seismostratigraphy in the interpretation of some regional lines. These seismic lines were acquired recently by a special survey program that allowed having good quality data until more than 35 km in the lithosphere (continental and transitional crust). The data permitted to interpret tectonic events such as the presence of Seawards Dipping Reflectors (SDR) in the transitional crust, covered by the Bengal Basin sediments. Besides the seismic interpretation tied to some well data, it was used in the dissertation a Beicip Franlab Software, known as Dionisos, to model and develop a basin fill history. The eustatic relative sea level flutuations and sediment rate of sediment influx are the key points to be considered in the model. Using the seismic data it was possible to recognize ten main platform breakes than were used in the model, tied to a geological time frame provided by the well data interpretation, from Pliocene to Holocene. The output of the model shown that a first half of the considered time predominated a major retrogradational system, with some minor trangression peaks. This changed to a very strong progradational system that is acting until the Holocene. The section modeled also shows a total of sediments deposited in the period considered was about 2,1 x106 km3 equivalent to a 9,41 x 1014 km3/Ma.
|
539 |
The sedimentology and palaeoenvironmental significance of vlei sediments on the Winterberg range, South AfricaDewey, Felicity Joy January 1989 (has links)
Palaeoenvironmental reconstruction of the late Pleistocene and Holocene geological periods for central and southern Africa has been hampered by the erratic distribution of suitable sites, incomplete and inaccurately dated sequences and the limited nature of published data. One geomorphological feature which has supplied valuable evidence for fluctuations in past environmental conditions, is the vlei or dambo. The type-site of these waterlogged features is in south central Africa, but similar features have been described on other continents. The clastic and organic sediments contained within these features are affected by, and therefore reflect to some degree, the environment under which they were formed. The characteristics of the sediments supply information as to their transport and mechanisms of deposition. From these processes, the environmental conditions at the time of vlei formation can be inferred. The environmental history of the Eastern Cape region has been considerably neglected, and is far less well understood than other countries such as Malawi, Zambia and Zimbabwe. A study site in the Winterberg Range (Eastern Cape) was selected which permitted the comparison of two vleis, the objective being to establish an accurate late Pleistocene sediment chronology for the entire plateau area. Radiocarbon dates from organic layers indicate that these sediments span the last 12 000 years BP, suggesting that organic accumulation at this site began at roughly the same time as at sites further afield. The vlei sediments are analysed in terms of their morphology, particle slze distribution, and other physical and chemical characteristics. These data facilitate the construction of detailed stratigraphic diagrams and a chronological summary of sediment accumulation, from which the period and governing processes of vlei development under changing environments may be described. It is found that the Winterberg vleis contain sediments which respond to changes in the prevailing environment. This makes these sediments useful indices from which to trace such changes during the late Pleistocene and Holocene times. These features are found to be similar in many respects to those described elsewhere in Southern Africa. The study attempts to provide greater understanding of contemporary vlei processes and emphasises the necessity of their preservation, as finite and valuable resources, by future generations
|
540 |
Modelagem sedimentar de sistemas deltaicos altamente eficientes: aplicação para o preenchimento sedimentar do terciário superior da bacia de Bengala, porção nordeste da Índia / High efficient deltaic system modeling: application for upper tertiary basin fill history of Bengal basin, Northeast IndiaMarlon Santos Delai 15 September 2011 (has links)
A bacia de Bengala, localizada a Nordeste da Índia tem uma história evolutiva extraordinária, diretamente controlada bela fragmentação do Gondwana. O início da formação desta bacia é considerada como sendo relacionada ao final do evento da quebra, datado em 126 Ma quando a Índia separou do continente Antártico e da Austrália. Desde então, a placa continental Indiana viajou do pólo sul a uma velocidade muito rápida (16 cm/a) chocando-se com o hemisfério norte e fundindo-se com a Placa Eurasiana. Durante a viagem passou por cima de um hot spot, onde hoje estão localizadas as ilhas Seicheles, resultando em um dos maiores derrames de lava basáltica do mundo, conhecido como Deccan Trap. Na região onde a bacia de Bengala foi formada, não houve aporte significativo de sedimentos siliciclásticos, resultando na deposição de uma espessa plataforma carbonática do Cretáceo tardio ao Eoceno. Após este período, devido a colisão com algumas microplacas e a amalgamação com a Placa Eurasiana, um grande volume sedimentar siliciclástico foi introduzido para a bacia, associado também ao soerguimento da cadeia de montanhas dos Himalaias. Atualmente, a Bacia de Bengala possui mais de 25 km de sedimentos, coletados neste depocentro principal. Nesta dissertação foram aplicados conceitos básicos de sismoestratigrafia na interpretação de algumas linhas regionais. As linhas sísmicas utilizadas foram adquiridas recentemente por programa sísmico especial, o qual permitiu o imageamento sísmico a mais de 35km dentro da litosfera (crosta continental e transicional). O dado permitiu interpretar eventos tectônicos, como a presença dos Seawards Dipping Reflectors (SDR) na crosta transicional, coberto por sedimentos da Bacia de Bengala. Além da interpretação sísmica amarrada a alguns poços de controle, o programa de modelagem sedimentar Beicip Franlab Dionisos, foi utilizado para modelar a história de preenchimento da bacia para um período de 5,2 Ma. O nível relativo do nível do mar e a taxa de aporte sedimentar foram os pontos chaves considerados no modelo.
Através da utilização dos dados sísmicos, foi possível reconhecer dez quebras de plataformas principais, as quais foram utilizadas no modelo, amarrados aos seus respectivos tempos geológico, provenientes dos dados dos poços do Plioceno ao Holoceno. O resultado do modelo mostrou que a primeira metade modelada pode ser considerada como um sistema deposicional retrogradacional, com algum picos transgressivos. Este sistema muda drasticamente para um sistema progradacional, o qual atuou até o Holoceno. A seção modelada também mostra que no período considerado o total de volume depositado foi em torno de 2,1 x 106 km3, equivalente a 9,41 x 1014 km3/Ma. / The Bengal Basin in Northeast India has a remarkable evolution history directly controled by the Gondwana fragmentation. The Beginning of this basin is considered related to the final break up event, dated as 126 Ma when India finally became separeted from Antarctica and Australia Continents. Since then, the India Continental Plate traveled from the South pole at very fast rate (16 cm/y) reaching the nortern hemisphere and merging with Eurasia Plate. During the travel it passed over a hot spot, located where is today Seychelles Island, resulting in one of the biggest basalt leakage, know as Deccan Trap. In the area where the Bengal Basin was formed during the travel there wasnt any supply of siliciclastics, resulting in the deposition of a very thick section of carbonate platforms from Late Cretaceous to Eocene. After this period, due to the collision with same microplates and the amalgamation with Eurasia Plate a huge siliciclastic supply was introduced in the basin, associated with rising of the Himalayas Mountains. Therefore, the Belgal Basin today has more than 25 km of sediments, collected in its main depocenter. In this dissertation it was applied to the basic concepts of seismostratigraphy in the interpretation of some regional lines. These seismic lines were acquired recently by a special survey program that allowed having good quality data until more than 35 km in the lithosphere (continental and transitional crust). The data permitted to interpret tectonic events such as the presence of Seawards Dipping Reflectors (SDR) in the transitional crust, covered by the Bengal Basin sediments. Besides the seismic interpretation tied to some well data, it was used in the dissertation a Beicip Franlab Software, known as Dionisos, to model and develop a basin fill history. The eustatic relative sea level flutuations and sediment rate of sediment influx are the key points to be considered in the model. Using the seismic data it was possible to recognize ten main platform breakes than were used in the model, tied to a geological time frame provided by the well data interpretation, from Pliocene to Holocene. The output of the model shown that a first half of the considered time predominated a major retrogradational system, with some minor trangression peaks. This changed to a very strong progradational system that is acting until the Holocene. The section modeled also shows a total of sediments deposited in the period considered was about 2,1 x106 km3 equivalent to a 9,41 x 1014 km3/Ma.
|
Page generated in 0.073 seconds