Gravity Fault Subsidence and Beach Ridges Progradation in Quinta-Cassino (RS) Coastal Plain, Brazil

Ground penetrating radar (GPR) surveys have being applied to investigate very near-surface stratification of sedimentary units in coastal plains and to define their depositional conditions. This paper presents, however, low-frequency GPR survey to investigate fault-related depositional systems at greater depths. The Quinta-Cassino area in the Rio Grande do Sul Coastal Plain (RGSCP, Brazil) shows a wide strandplain that is made off by very long, continuous, and linear geomorphic features (beach ridges). This strandplain extends for ~70 km southward. The beach ridges show low-angle truncations against the Quinta escarpment, and also truncations in the strandplain. The traditional approach points that RGSCP was developed by juxtaposition of four lagoons/barrier systems as consequence of sea level changes;previous model assumes that no deformational episode occurred in RGSCP. The geophysical and geological surveys carried out in this area showed the existence of listric fault controlling the beach ridges in the escarpments and hanging-wall blocks. The radargrams could distinguish Pleistocene basement unit anticlockwise rotation, thickening of beach ridges radarfacies close to listric normal faults, and horst structures. These deformational features indicate that the extensional zone of a large-scale gravity-driven structure controlled the mechanical subsidence, the Holocene sedimentation and its stratigraphic and geomorphic features in the Quinta-Cassino area to build up an asymmetric delta. The results point to a new approach in dealing with RGSCP Holocene evolution.

Progradation of the Changjiang River delta since the mid-Holocene

Subaqueous deltaic deposits with approximately 30 radiocarbon ages show that the Changjiang River delta was strongly affected by tides and that the delta progradation rate after 2 kaBP was almost double the rate before 2 kaBP. This change in the progradation rate correlates well with the active extension of the subaerial delta plain shown by previous work. Widespread human activities, such as farming, deforestation, and dike construction, probably resulted in an increase in sediment discharge to the river-mouth area.

Sequence Stratigraphic Analysis of “XB Field”, Central Swamp Depobelt, Niger Delta Basin, Southern Nigeria

Well logs and biostratigraphic data from six wells in the “XB Field”, central Swamp Depobelt, Niger Delta were integrated to carry out a sequence stratigraphic analysis of depositional systems in the field. The analysis revealed four 3rd order depositional sequences (SEQ1 to 4) bounded by three erosional unconformities interpreted as Sequence Boundaries (SB1 to 3). Transgressive Surfaces of Erosion (TSE1 to 3) that mark the onset of marine flooding and turnarounds from progradational facies to retrogradational facies during sequence build-up were delineated. Three 3rd order Maximum Flooding Surfaces (MFS1, MFS2 and MFS3) characterized by marker shales, high faunal abundance and diversity were also delineated and dated 15.9, 17.4 and 19.4 Ma, respectively. The delineated sequences comprised Lowstand Systems Tracts (progradational packages), Transgressive Systems Tracts (retrogradational packages) and Highstand Systems Tracts (aggradational packages), which reflect depositional systems deposited during different phases of base level changes. The Lowstand Systems Tract (LST) consists of Basin Floor Fans (BFF), Slope Fans and Channel Sands deposited when sea level was low and accommodation space lower than rate of sediment influx. Transgressive Systems Tract (TST) consists of retrogradational marine shales deposited during high relative sea levels and when accommodation space was higher than rate of sediment influx. Highstand Systems Tracts (HST) consisted of shoreface sands displaying mostly aggradational to progradational stacking patterns. The sands of LST and HST show good reservoir qualities while the shales of the TSTs could form potential reservoir seals. The above recognized sequences, were deposited within the Neritic to Bathyal paleoenvironments and are dated mid-Miocene (15.9 - 20.4 Ma) in age.

New constraints on the P-T path of HT/UHT metapelites from the Highland Complex of Sri Lanka

We report here rare evidence for the early prograde P-Tevolution of garnet-sillimanite-graphite gneiss(khondalite)from the central Highland Complex,Sri Lanka.Four types of garnet porphyroblasts(Grt_1,Grt_2,Grt_3 and Grt_4)are observed in the rock with specific types of inclusion features.Only Grt_3 shows evidence for non-coaxial strain.Combining the information shows a sequence of main inclusion phases,from old to young:oriented quartz inclusions at core,staurolite and prismatic sillimanite at mantle,kyanite and kyanite pseudomorph,and biotite at rim in Grt_1;fibrolitic sillimanite pseudomorphing kyanite±corundum,kyanite,and spinel+sillimanite after garnet+corundum in Grt_2;biotite,sillimanite,quartz±spinel in Grt_3;and ilmenite,rulite,quartz and sillimanite in Grt_4.The pre-melting,original rock composition was calculated through stepwise re-integration of melt into the residual,XRF based composition,allowing the early prograde metamorphic evolution to be deduced from petrographical observations and pseudosections.The earliest recognizable stage occurred in the sillimanite field at around 575℃ at 4.5 kbar.Subsequent collision associated with Gondwana amalgamation led to crustal thickening along a P-T trajectory with an average dP/dT of ~30 bar/℃ in the kyanite field,up to ~660℃ at 6.5 kbar,before crossing the wet-solidus at around 675 ℃ at 7.5 kbar.The highest pressure occurred at P > 10 kbar and T around 780℃ before prograde decompression associated with further heating.At 825℃ and 10.5 kbar,the rock re-entered into the sillimanite field.The temperature peaked at 900℃ at ca.9-9.5 kbar.Subsequent near-isobaric cooling led to the growth of Grt_4 and rutile at T ~880℃.Local pyrophyllite rims around sillimanite suggest a late stage of rehydration at T<400℃,which probably occurred after uplift to upper crustal levels.U-Pb dating of zircons by LAICPMS of the khondalite yielded two concordant ^(206)Pb/^(238)U age groups with mean values of 542±2 Ma(MSWD=0.24,Th/U=0.01-0.03)and 514±3 Ma(MSWD=0.50,Th/U=0.01-0.05)interpreted as peak metamorphism of the khondalite and subsequent melt crystallization during cooling.

Geochemistry and geochronology of multi-generation garnet:New insights on the genesis and fluid evolution of prograde skarn formation

Origin of garnet in skarn(magmatic vs.hydrothermal)and the prograde skarn fluid evolution are still controversial.Two generations of garnet(Grt1,Grt2)were identified at the Tongshankou deposit:Grt1 is anisotropic with oscillatory zoning and resorbed boundary,whilst Grt2 grew around Grt1 and formed oscillatory rims.In-situ LA-ICP-MS U-Pb dating of Grt1 and Grt2 yielded a lower intercept^(206)Pb/^(238)U age of 142.4±2.8 Ma(n=57;MSWD=1.16)and 142.3±9.6 Ma(n=60;MSWD=1.06),respectively,coeval with the ore formation and ore-related granodiorite emplacement.Positive Eu anomaly,non-CHARAC Y/Ho value and low TiO_(2)content,together with the mineral assemblages indicate that both Grtl and Grt2 have a hydrothermal origin.The existence of melt and melt-fluid inclusions in Grt1,together with similar LREE-enriched patterns to the granodiorite,further indicate that Grt1 may have formed in the magmatic-hydrothermal transition.Higher U contents and LREE-enriched patterns of Grt1 indicate that fluid I is mildly acidic pH and low fO_(2).The inner gray Grt2 rims(Grt2A)is HREE-enriched with low U contents,indicating that fluid II has nearly neutral pH and high fO_(2).The wider Y/Ho range and LREE-enriched patterns of the outer light-gray Grt2 rims(Grt2B)show that the evolved magmatic fluidⅡhad mixed with an external fluid,characterized by being mildly acidic pH and with high fO_(2)·Our results suggest that the prograde skarn-forming fluids can be multistage at Tongshankou,and the mixing of meteoric water may have been prominent in the prograde skarn stage.

The response of deltaic systems to climatic and hydrological changes in Daihai Lake rift basin,Inner Mongolia,northern China

Delta systems are ubiquitous around lacustrine rift basins. Its peripheral ge-ometry, progradation structures and sedimentary successions were controlled by both tec-tonic settings and climatic changes. Peripheral geometry of a delta was strongly influenced by depositional gradients which formed the fan-shape delta on the steep slopes and developed the lobe-shape delta on the gentle slopes. Due to the discharge feed rivers can change rapidly driven by climatic variations, and the nearshore area of deltas display considerable facies variability. The rise of annual rainfall, which suggests the rivers feeding deltas are continuous, and result in distributary mouth bars that are prevalent in the front of deltas since the down-slope flows are greater than the along-slope currents. On the contrary, when the annual rainfall decreases and evaporation increases, the rivers only can feed deltas ephemerally. If the along-slope currents were in a dominant position, the distal bars were deposited. Progra-dation structure and sedimentary successions of deltas were controlled by the gradients of slopes. On gentle depositional slopes, shingle foreset beds predominate with fine sediments and small-scale sedimentary structures or vice versa.

Temperature of Prograde Metamorphism, Decompressional Partial Melting and Subsequent Melt Fractional Crystallization in the Weihai Migmatitic Gneisses,Sulu UHP Terrane:Constraints from Ti-in-Zircon Thermometer

In order to constrain temperature during subduction and subsequent exhumation of fel- sic continental crust, we carried out a Ti-in-zircon thermometer coupled with zircon internal structure and U-Pb age on migmatitic gneisses from the Weihai region in the Sulu ultra-high pres- sure （UHP） metamorphic terrane, eastern China. The Weihai migmatitic gneisses are composed of in- tercalated compositional layers of melanosome and plagioclase （Pl）-rich lencosome and K-feldspar （Kfs）-rich pegmatite veins. Four stages of zircon growth were recognized in the Weihai migmatitic gneisses. They successively recorded informations of protolith, prograde metamorphism, decompres- sional partial melting during early stage exhumation and subsequent fractional crystallization of pri- mary melt during later stage cooling exhumation. The inherited cores in zircon from the melanosome and the Pl-rich leucosome suggest that the pro- tolith of the migmatitic gneiss is Mid- Neoproterozoic （-780 Ma） magmatic rock. Metamorphic zircons with concordant ages ranging from 243 to 256 Ma occur as over- growth mantles on the protolith magmatic zir- con cores. The estimated growth temperatures （625-717 ＂C） of the metamorphic zircons have a negative correlation with their ages, indicating a progressive metamorphism in HP eciogite-facies condition during subduction. Zircon recrystal- lized rims （228-2 Ma） in the PI-rich ieucosome layers provide the lower limit of the decompress-sional partial melting time during exhumation. The ages from 228^-2 to 219~2 Ma recorded in the Pl-rich leucosome and the Kfs-rich pegmatite vein, respectively, suggest the duration of the fractional crystallization of primary melt during exhumation. The calculated growth temperatures of the zircon rims from the Pl-rich leucosome range from 858 to 739 , and the temperatures of new growth zircon grains （219±2 Ma） in Kfs-rich vein are between 769 and 529 . The estimated temperatures have a positive correlation with ages from the Pl-rich leucosome to the Kfs-rich pegmatite vein, strongly indi- cating that a process of fractional crystallization of the partial melt during exhumation.

Hyperpycnal littoral deltas:A case of study from the Lower Cretaceous Agrio Formation in the Neuquén Basin,Argentina

Recent advances in the understanding of deltaic deposits provide new tools for the study and analysis of deltaic deposits in shallow epicontinental seas.After the introduction of sequence stratigraphic concepts,meter-scale coarsening and thickening upward successions have been considered as“parasequences”originated by high-frequency sea-level changes.Nevertheless,recent studies enhanced the importance of wave-aided low-dense hyperpycnal flows in transporting fine-grained sediments in shallow shelfal areas.These poorly-known(and at the same time very common)types of delta,known as hyperpycnal littoral deltas(HLD),develop very low gradient progradational units,controlled by changes in the sediment supply instead of sea level changes.These small-scale progradational units are very common in shallow epicontinental seas like the Lower Cretaceous Agrio Formation in the Neuquén Basin.This study provides a first detailed analysis of hyperpycnal littoral deltas from the Agua de la Mula Member(upper Hauterivian-lower Barremian)of the Agrio Formation.This unit has been studied in three locations near Bajada del Agrio locality in the central part of the Neuquén Basin,Argentina.Six sandy facies,three heterolithic facies,three muddy facies and four calcareous facies were recognized.From facies analysis,three facies associations could be determined,corresponding to offshore/prodelta,distal ramp delta and proximal ramp delta.The three stratigraphic sections discussed in this study are internally composed of several small-scale sequences showing a coarsening and thickening upward pattern,transitionally going from muddy to sandy wave-dominated facies,and ending with calcareous bioclastics levels on top.These small-scale sequences are interpreted as delta front deposits of wave-influenced hyperpycnal littoral deltas,punctuated by calcareous intervals accumulated during periods of low sediment supply.It is interpreted that the development of hyperpycnal littoral deltas could have been facilitated by a decrease in sea water salinity related to an increasing runoff.