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Introduction To Coastal Processes And Geomorphology

This paper presents an overview of some of the most significant, recent to ancient, littoral morpho-sedimentary structures and deposits from the Lake Turkana Basin. We highlight the importance of wave-related sedimentary processes in lakes, and more specifically in rift lakes. In the published literature, references to wave-dominated shorelines are mainly in regards to coastal marine environments. However, numerous modern lakes exhibit typical wave-dominated littoral landforms, and related sedimentary deposits are known from several paleolake successions in the geological record. Wave-related processes are often of relatively minor importance in depositional models for lacustrine environments. Classical models emphasize clastics transported by rivers, which are then distributed by fan-deltas and/or deltas into a water body of fluctuating depth, where reworking of clastics is limited in the littoral domain, and episodic in deep waters. Modern processes in Lake Turkana and the exposed paleolake deposits of the Turkana Basin demonstrate that this view is incomplete. Wave-dominated shorelines are evident (1) for modern Lake Turkana based on prominent and active littoral landforms (e.g., beach ridges, sand spits, washover fans, and arcuate-cuspate deltas); (2) for the Holocene (African Humid Period) climate-driven highstand of Megalake Turkana and its subsequent forced regression based on conspicuous raised beach ridges and spits; and (3) for the Pliocene–Pleistocene (Omo Group, Nachukui Formation) from typical nearshore sedimentary facies and stratigraphic architectures associated with paleolake Turkana. These examples from the Turkana Basin coupled with examples from other lacustrine settings, suggest that wave-dominated clastic shorelines represent significant portions of existing and ancient lake-shores. As this view contrasts with classic depositional models for lakes, notably for those found in rift setting, we also present examples of wave-influenced littoral landforms from other lakes of the East African Rift System. Identifying lacustrine paleoshorelines from typical clastic landforms and deposits is the key to the spatial reconstruction of lakes over time, and to determine transgressive–regressive cycles. Waves action is an important agent in lakes for the erosion, transport, and deposition of clastics at the basin-scale, an aspect that needs to be integrated in sedimentary models.

Lacustrine wave-dominated clastic shorelines: modern to ancient littoral landforms and deposits from the Lake Turkana Basin (East African Rift System, Kenya)

Orbitally-driven evolution of Lake Turkana (Turkana Depression, Kenya, EARS) between 1.95 and 1.72 Ma: A sequence stratigraphy perspective

Lakes are common natural systems for which sedimentation is considered to be relatively simple, generally dominated by fluvial processes along the margin and prevailing low-energy settling in the central, deeper parts. However, for many lakes, higher-energy wind-driven processes dominate. As such, a new category of lakes is proposed, herein referred to as wind-driven waterbodies (WWB). WWB display a sedimentation largely dominated by wave related processes and wind-induced lake-scale water circulation evidenced by the construction of beach ridges, spits or cuspate spits along their shorelines, and by sediment drifts, sedimentary shelf progradation and erosional surfaces in their deeper, offshore domains. WWB are observed worldwide, they share a common physiography that favours wind-forced hydrodynamics and related sedimentation patterns. This physiography is expressed by the IWWB index, a ratio of the maximum representative fetch relative to mean basin depth. It is proposed that an index value greater than three favours the evolution of a lake as a WWB. The WWB concept represents a new end-member in an alternative, sedimentologically-based lake classification that is proposed in this paper.

Wind-driven waterbodies: a new category of lake within an alternative sedimentologically-based lake classification

The Ground Penetrating Radar facies and architecture of a Paleo-spit from Huangqihai Lake, North China: Implications for genesis and evolution

Soil and rock mixture (SRM) consists of high-strength rock blocks and small-grained soils, which depend seriously on the rock size and the heterogeneity of structure. Because of inhomogeneous cementation between rock blocks, complex granulometric composition, and random distribution of rock blocks, these SRMs usually cause problems. Investigation of the localized deformation is critical for successful engineering designs, engineering safety assessment. In this work, uniaxial compression testing on the SRM with a rock block percentage of 40% (mass ratio) is performed under real-time computed tomography (CT) combined with digital image correlation (DIC) technology. Based on radial strain fields and shear fields, heterogeneity of strain fields and rock block motion is analyzed quantitatively by semi-variance. The results show that rock block is the main factor controlling the shear band geometry distribution. A localization strain band usually occurs along the soil and rock interfaces. From the experimental results by the DIC method, the damage factor is presented to describe the damage evolution of the SRM under uniaxial compression. The method proposed in this study can be used to quantitatively analyze the deformation characteristics of the SRM sample.

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Mingtao Li

Papers

The Ground Penetrating Radar facies and architecture of a Paleo-spit from Huangqihai Lake, North China: Implications for genesis and evolution

Analysis on Spatial Variability of SRM Based on Real-Time CT and the DIC Method Under Uniaxial Loading

High-resolution Early Triassic ammonoid biostratigraphy of South Tibet, China and implications for global correlations