Formation of box canyons by mass failure in limestone: A modelling study of the role of groundwater seepage

<p>Groundwater seepage has been shown to unambiguously lead to channel formation inunconsolidated sand to gravel sized sediments. However, its role in the evolution of bedrocklandscapes remains controversial. In this study, we use the coastline of the Maltese Islands as a case study to establish if and how groundwater seepage can form box canyons in limestones. The study area comprises up to 40 m high coralline limestone cliffs, with a mean fracturedensity of 1 in 5 m, overlying a ductile marl layer. The permeability contrast promotes the development of a perched aquifer and groundwater seepage at the cliff face. We  ran numerical simulations using a 3D distinct element model based on geological, geotechnical and hydrological baseline information from the study state, and explored three potential mechanisms: (i) fracture widening by fluid pressure and dissolution associated to groundwater flow and seepage, (ii) fracture widening by loss of support at the base due tomarl displacement resulting from increased water content, and (iii) a combination of (i) and (ii). We also took into consideration two scenarios: (a) uniform groundwater seepage, and (b)focused groundwater seepage. Our results suggest that the combination of mechanisms (iii) and the scenario with focused groundwater seepage (b) give rise to the box canyonmorphology observed at the site. Box canyons thus initiate and grow via detachment of limestone blocks and their toppling, which is more concentrated at the head where groundwater seepage occurs.</p>

Role of Microbial Communities in the Weathering and Stalactite Formation in Karst Topography

This study investigated the long-term effect of environmental physical factors on the relative abundance of bacteria and the consequential landscape evolution in karst topography, focusing mainly on the effects of limestone weathering and calcite precipitation. The Narrow-Sky located in the upper part of Takangshan is a small gulch of Pleistocene coralline limestone formation in southern Taiwan. The landscapes were different in the karst walls between the opening and the inner of gulch due to the variation of physical parameters such as sunlight penetration, humidity, and temperature. A metagenomic approach was used out to determine the relationship of microbial community structures on the landscapes in various habitats around the gulch, namely on the inner and outer limestone wall, the water collected from speleothems surface, and the ground soil at the outer wall. The total organic carbon content was measured in solid samples to evaluate the biomass of the habitats. Our results showed that the biomass of habitats in the opening of the gulch was two times higher than the that inside where light penetration was lower. We also found that speleothems only occurred at the inner wall inside the gulch, where the environment exhibited water drips running through the surface of speleothems and less light penetration. The metagenomics in each habitat was surveyed to measure the sequence similarity of operational taxonomic units relative to urease-producing bacteria and weathering-associated bacteria available in the National Center for Biotechnology Information database. Our data revealed that the metagenomics of the inner wall and water samples exhibited more sequences that were similar to those of urease-producing bacteria, whereas the outer wall showed more sequences that were similar to those of weathering-associated bacteria, suggesting that bacteria facilitated the formation of limestone weathering and calcite precipitation for various habitats. This study revealed the pivotal roles of microorganisms in governing the geological evolution of the limestone landscape.

PALEONTOLOGY OF ACROPORA CORALS AND STANDARD FACIES BELT FROM UJUNGGENTENG AREA, WEST JAVA

The detail taxonomy analysis was performed to classify Acropora corals in Ujunggenteng Area. The research area was selected because the continuously exposed Quaternary coralline limestones, indicated the high variation and wide distribution of coral fossils. Moreover, the facies changes and contacts with shoreface sediments were clearly observed in this area. Detail taxonomy based on morphological description can classify Acropora corals in Ujunggenteng area into four species: Acropora cervicornis, Acropora palifera, Acropora gemmifera, and Acropora humilis. The study of coral paleontology and the application of the presence of corals as a standard facies belt were still rarely performed in Indonesia. Previous studies classified the coralline limestone into one standard facies belt, which was the organic build- up standard facies belt. Another approach was required to capture many conditions of coral fossil occurrences; not only in build-up condition but also in transported condition. Therefore, another purpose of this study is to modify the standard facies belt with a different approach using coral taphonomy and sediment association.Analisis taksonomi secara detil dilakukan untuk mengklasifikasikan koral Acropora di daerah Ujunggenteng. Daerah penelitian dipilih karena tersingkapnya batugamping terumbu berumur Kuarter yang menerus, yang menunjukkan tingginya jumlah spesies dan distribusi fosil koral yang luas. Selain itu, perubahan fasies dan kontak dengan batupasir pantai dapat jelas diamati pada daerah ini. Taksonomi detil berdasarkan deskripsi morfologi dapat mengelompokkan koral Acropora di daerah Ujunggenteng menjadi empat spesies: Acropora cervicornis, Acropora palifera, Acropora gemmifera, dan Acropora humilis. Selain itu, studi mengenai paleontologi dan penggunaan kehadiran koral sebagai dasar pembagian sabuk standar fasies batugamping masih jarang dilakukan di Indonesia. Studi sebelumnya mengelompokkan batugamping terumbu menjadi satu sabuk standar fasies, yaitu organic build up. Pendekatan yang lain diperlukan untuk menjelaskan kondisi koral lainnya pada batugamping, tidak hanya dalam kondisi tumbuh, tetapi juga dalam kondisi tertransportasi. Oleh karena itu, penelitian ini bertujuan untuk memodifikasi sabuk standar fasies dengan pendekatan berbeda menggunakan tafonomi koral dan asosiasi sedimen.