To determine the characteristics and potential indicators of modern typhoon deposition in a sandy lagoon off the coast of Guangdong Province (southern China), we analysed the 210Pb, sedimentology, and microfossils of samples from ten cores obtained before and after the passage of Typhoon Rammasun in 2014. Typhoon deposition showed a thinning trend from internal areas of the lagoon to its mouth, with the maximum thickness inside the lagoon of ~35 cm. These typhoon deposits are dominated by overwash and differ from sediments deposited under normal weather conditions. Under normal weather conditions, lagoon sediment has a 210Pb curve that follows a model of exponential decay, has a unimodal granularity frequency curve, and lacks organic matter and microfossils (diatoms and foraminifera). However, 210Pb is low in the typhoon deposits, the grain size is coarse, and the granularity frequency curve is obviously bimodal. There are also abundant foraminifera in the typhoon deposits. We found a clear double-layered structure in the typhoon deposits, which was caused by strong hydrodynamic disturbance that mixed sediments originally from the offshore area with those of the lagoon. The lower layer has coarse-grained particles with medium sorting, low organic matter content, and low diatom content. The upper layer has fine-grained particles with poor sorting, high organic matter content, and abundant diatoms. The rate of fragmentation of diatoms in the upper layer was very high (40%-60%). The diatom assemblage contained offshore and freshwater species carried by storm runoff. Therefore, we believe that the sediments of this typical sand bar-lagoon environment retain evidence of typhoon events along the southern China coast that is displayed in the marked sedimentological and microfossil characteristics.
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.
Based on the analysis of element geochemistry and total organic carbon (TOC), this study investigates the main factors controlling organic matter (OM) enrichment, reconstructs the evolution process of the sedimentary environment, and proposes a dynamic OM enrichment model of the Jurassic Da’anzhai (D) Formation, Sichuan Basin. The results indicate that the Sichuan Basin was generally dominated by a warm and oxidizing sedimentary environment, but with some peculiarities, such as a hotter climate in the D1 member and more anoxic lake water in the D2a member. The sedimentary evolution of the Da’anzhai Formation can be divided into a fluctuating sedimentary stage, a stable sedimentary stage and a reef-building stage. The D2a member showed the strongest hypoxia, the weakest weathering, the largest amount of terrestrial inputs, and the highest TOC content. The TOC is positively correlated with reducing conditions and terrestrial inputs, negatively correlated with weathering. Based on these findings, it is suggested that the global climate in the Early Jurassic period had a complex regional effect and the global oceanic anoxic events of the Toarcian did not spread to the Sichuan Basin. Thus, the anoxic deep water, high terrestrial inputs, and weak weathering were conducive to rapid deposition and preservation of lacustrine OM.
The Cuddalore Formation of the Cauvery Basin received siliciclastic detritus from inland areas of the Southern Granulite Terrain (SGT). It represented continental-fluvial sedimentation in the eastern continental margin of South India during the Miocene. Indian Summer Monsoon was thought to be initiated in the early Miocene and intensified during the middle Miocene causing major climatic shifts in the Indian subcontinent. In the present work, detailed mineralogical and geochemical studies on the siliciclastic Cuddalore Formation have been carried out to understand the provenance and paleoclimatic conditions during the Miocene. The paleocurrent direction, textural immaturity and framework detrital modes of sandstones suggest rapid uplift of basement and sediment source from nearby Madras Block of SGT. Various diagnostic immobile trace element ratios such as Th/Sc, Co/Th, La/Sc, La/Co suggest a tonalite-trondhjemite-granodiorite-charnockite provenance, and somewhat more felsic composition of source area compared to the present upper continental crust (UCC). Rare earth element mixed model suggests that sediments were dominantly (80%) sourced from felsic charnockite, with a minor contribution (20%) from mafic granulites. Higher abundance of advanced-weathering products like kaolinite, very high (>98) chemical index of alteration (CIA) values, severe depletion of mobile elements (Ca, Na, K, Mg, Ba, Rb) in comparison to UCC, significantly higher αMg, αCa, αNa, αK, αSr and αBa values (higher than the unity), all suggest the extreme chemical weathering in source terrain and/or in the sedimentary basin. Calculations based on CIA show high average surface temperature between 29.3 °C and 29.5 °C and high mean annual precipitation ranging from 2339 mm/yr to 2467 mm/yr. The geochemical data are consistent with the paleogeographic position of the depositional basin (Cauvery Basin) and suggest the deposition of Cuddalore sediments (the Cuddalore Formation) in a near-equatorial location under a warm climate condition with abundant monsoonal precipitation.
