Geometrical Breakdown Approach to interpretation of depositional sequences

Seismic and sequence stratigraphic analyses are important methodologies for interpreting coastal and shallow-marine deposits. Though both methods are based on objective criteria, terminology for reflection/stratal stacking is widely linked to eustatic cycles, which does not adequately incorporate factors such as differential subsidence, sediment supply, and autogenic effects. To reduce reliance on model-driven interpretations, we developed a Geometrical Breakdown Approach (GBA) that facilitates interpretation of horizon-bound reflection packages by systematically identifying upward-downward and landward-seaward trajectories of clinoform inflection points and stratal terminations, respectively. This approach enables a rigorous characterization of stratal surfaces and depositional units. The results are captured in three-letter acronyms that provide an efficient way of recognizing repetitive stacking patterns through discriminating reflection packages objectively to the maximum level of resolution provided by the data. Comparison of GBA with selected sequence stratigraphic models that include three and four systems tracts and the accommodation succession approach shows that the GBA allows a greater level of detail to be extracted, identifying key surfaces with more precision and utilizing more effectively the fine-scale resolution provided by the input seismic data. We tested this approach using a synthetic analogue model and field data from the New Jersey margin. The results demonstrate that the geometric criteria constitute a reliable tool for identifying systems tracts and provide an objective and straightforward method for practitioners at all levels of experience

Incised valley paleoenvironments interpreted by seismic stratigraphic approach in Patos Lagoon, Southern Brazil

<div><p>ABSTRACT: The Rio Grande do Sul (RS) coastal plain area (33,000 km 2 ) had its physiography modified several times through the Quaternary, responding to allogenic and autogenic forcings. The Patos Lagoon covers a significant area of RS coastal plain (10,000 km 2 ), where incised valleys were identified in previous works. About 1,000 km of high resolution (3.5 kHz) seismic profiles, radiocarbon datings, Standard Penetration Test (SPT) and gravity cores were analyzed to interpret the paleoenvironmental evolution as preserved in incised valley infills. Seismic facies were recognized by seismic parameters. The sediment cores were used to ground-truth the seismic interpretations and help in the paleoenvironmental identification. Key surfaces were established to detail the stratigraphical framework, and seismic facies were grouped into four seismic units, which one classified in respective system tracts within three depositional sequences. The oldest preserved deposits are predominantly fluvial and estuarine facies, representing the falling stage and lowstand system tracts. The Holocene transgressive records are dominated by muddy material, mainly represented by estuarine facies with local variations. The transgression culminated in Late Holocene deposits of Patos Lagoon, representing the highstand system tract. The depositional pattern of the vertical succession was controlled by eustatic variations, while the autogenic forcing (paleogeography and sediment supply) modulated the local facies variation.</p></div

Modeling the interactions between river morphodynamics and riparian vegetation

The study of river-riparian vegetation interactions is an important and intriguing research field in geophysics. Vegetation is an active element of the ecological dynamics of a floodplain which interacts with the fluvial processes and affects the flow field, sediment transport, and the morphology of the river. In turn, the river provides water, sediments, nutrients, and seeds to the nearby riparian vegetation, depending on the hydrological, hydraulic, and geomorphological characteristic of the stream. In the past, the study of this complex theme was approached in two different ways. On the one hand, the subject was faced from a mainly qualitative point of view by ecologists and biogeographers. Riparian vegetation dynamics and its spatial patterns have been described and demonstrated in detail, and the key role of several fluvial processes has been shown, but no mathematical models have been proposed. On the other hand, the quantitative approach to fluvial processes, which is typical of engineers, has led to the development of several morphodynamic models. However, the biological aspect has usually been neglected, and vegetation has only been considered as a static element. In recent years, different scientific communities (ranging from ecologists to biogeographers and from geomorphologists to hydrologists and fluvial engineers) have begun to collaborate and have proposed both semiquantitative and quantitative models of river-vegetation interconnections. These models demonstrate the importance of linking fluvial morphodynamics and riparian vegetation dynamics to understand the key processes that regulate a riparian environment in order to foresee the impact of anthropogenic actions and to carefully manage and rehabilitate riparian areas. In the first part of this work, we review the main interactions between rivers and riparian vegetation, and their possible modeling. In the second part, we discuss the semiquantitative and quantitative models which have been proposed to date, considering both multi- and single-thread river

Discontinuity-bounded alluvial sequences of the southern gangetic plains, India: Aggradation and degradation in response to monsoonal strength

ABSTRACT: Discontinuity-bounded late Quaternary sequences in the southern Gangetic Plains (Himalayan Foreland Basin) reflect flood-plain aggradation and degradation in response to forcing by the pow-erful Southwest Indian Monsoon. The major Himalayan and cratonic rivers in this area occupy narrow, incised valleys and do not inundate the adjoining broad interfluves, which display soil development, small plains-fed rivers, lakes, eolian deposits, and badland ravines. However, these areas formerly experienced active alluviation because thick flood-plain muds underlie the interfluve. An age model suggests that inter-fluve areas near the major rivers aggraded periodically between about 27 ka and 90 ka (Marine Oxygen Isotope Stages 3–5). They subse-quently degraded or accumulated sediment only locally, probably re-flecting decreased monsoonal precipitation around the Last Glacial Maximum (Marine Isotope Stage 2), when major river valleys moved to an underfit condition. Increased precipitation during the 15 to 5 k

Erosional marks on consolidated banks and slump blocks in the Rupen River, North-West India

Erosional marks, including grooves, current crescents and flutes, are present on banks and slump blocks of the Rupen River on the semi-arid plains of north-west India. The channel has a width:depth ratio of 6, and is incised up to 8 m through well consolidated, fine-grained aeolian sediment dated at ˜20-60 ka. Pedogenic carbonate nodules embedded in the channel walls have localized scour, and reworked carbonate particles probably promoted corrasion during strong seasonal flow. Similar erosional marks are apparently scarce on modern river banks, and a distinctive combination of conditions, including a consolidated substrate and strong flow in a confined channel, is probably required for their generation and preservation on channel walls and on slump blocks

Calcretes at sequence boundaries in upper carboniferous cyclothems of the Sydney Basin, Atlantic Canada

Nodular and associated groundwater calcretes in coal-bearing cyclothems of the Sydney Basin mark the boundary between underlying marginal-marine and overlying alluvial deposits. They are inferred to represent lowstand surfaces (sequence boundaries). The host sediments include both sand/siltstones and palustrine limestones. Microfabrics of calcretes with detrital hosts show evidence of replacement, displacement and shrinkage (alpha fabrics), suggesting that the calcretes formed under relatively arid conditions. In contrast, microfabrics of nodular calcretes with limestone hosts exhibit predominantly beta fabrics, with root-induced brecciation and subordinate shrinkage fabrics. The calcretes and host limestones are inferred to have formed in topographic lows. Thick alluvial sediments with red calcic vertisols overlie the calcretes, and formed within the transgressive systems tract under conditions of abundant sediment supply. Coals and associated hydromorphic palaeosols lie near the transgressive maximum and in the highstand systems tract. The presence of calcareous palaeosols on lowstand surfaces suggests that the lowstands were times of relative climatic aridity