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An old guano pile deposited by Macroderma gigas has been examined chemically and mineralogically. Sixteen sectional analyses of a profile are presented and discussed. The main soluble components are P2O5, CaO and SO3 (present as brushite, ardealite, gypsum and collophane) with insoluble quartz. Taranakite occurs as a minor constituent.
The general objective of this work was to develop a basic understanding of the karst hydrology, the nature and origin of the caves, the water chemistry, the surface geomorphology, and relationships among these aspects for a high relief tropical karst region having a thick section of limestone. The Valles-San Luis Potosí region of northeastern México, and in particular, the Sierra de El Abra, was selected for the study. A Cretaceous Platform approximately 200 km wide and 300 km long (N-S) delimits the region of interest. A thick Lower Cretaceous deposit of gypsum and anydrite, and probably surrounded by Lower Cretaceous limestone facies, is overlain by more than 1000 m of the thick-bedded middle Cretaceous El Abra limestone, which has a thick platform-margin reef. The Sierra de El Abra is a greatly elongated range along the eastern margin of the Platform. During the late Cretaceous, the region was covered by thick deposits of impermeable rocks. During the early Tertiary, the area was folded, uplifted, and subjected to erosion. A high relief karst having a wide variety of geomorphic forms controlled by climate and structure has developed. Rainfall in the region varies from 250-2500 mm and is strongly concentrated in the months June-October, when very large rainfalls often occur.
A number of specific investigations were made to meet the general objective given above, with special emphasis on those that provide information concerning the nature of ground-water flow systems in the region. Most of the runoff from the region passes through the karstic subsurface. Large portions of the region have no surface runoff whatsoever. The El Abra Formation is continuous over nearly the whole Platform, and it defines a region of very active ground-water circulation. Discharge from the aquifer occurs at a number of large and many small springs. Two of them, the Coy and the Frío springs group, are among the largest springs in the world with average discharges of approximately 24 m³/sec and 28 m³/sec respectively. Most of the dry season regional discharge is from a few large springs at low elevations along the eastern margin of the Platform. The flow systems give extremely dynamic responses to large precipitation events; floods at springs usually crest roughly one day after the causal rainfall and most springs have discharge variations (0max/0min) of 25-100 times. These facts indicate well-developed conduit flow systems.
The hydrochemical and hydrologic evidence in combination with the hydrogeologic setting demonstrate the existence of regional ground-water flow to several of the large eastern springs. Hydrochemical mixing-model calculations show that the amount of regional flow is at least 12 m³/sec, that it has an approximately constant flux, and that the local flow systems provide the extremely variable component of spring discharge. The chemical and physical properties of the springs are explained in terms of local and regional flow systems.
Local studies carried out in the Sierra de El Abra show that large conduits have developed, and that large fluctuations of the water table occur. The large fossil caves in the range were part of great deep phreatic flow systems which circulated at least 300 m below ancient water tables and which discharged onto ancient coastal plains much higher than the present one. The western margin swallet caves are of the floodwater type. The cave are structurally controlled.
Knowledge gained in this study should provide a basis for planning future research, and in particular for water resource development. The aquifer has great potential for water supply, but little of that potential is presently used.
Investigation of two King Island sea caves developed in quartzitic rocks shows them to contain a wealth of clastic and chemical sediments. Clastic sediments consist of wave-rounded cobbles, debris cones, and angular rock fragments produced by frost weathering and crystal wedging. Chemical deposits include a variety of calcium carbonate speleothems and also gypsum occurring as wall crusts and blisters. The latter appear to be a speleothem type of rare occurrence. Growth of gypsum is responsible for some crystal wedging of the bedrock. Three basal stalagmite samples have been dated by the Th/U method indicating Late Pleistocene as well as Holocene speleothem growth. The caves are believed to have formed by preferential wave erosion during the Last Interglacial in altered and fractured quartzites. The evidence for pre-Holocene evolution of sea caves and geos in the Tasman region is summarised. Tasmania and the Bass Strait Islands provide a particularly favourable environment for the preservation of relict landforms on rocky coasts because of Late Quaternary uplift. The potential of further studies of sea caves to test two recently advanced archaeological hypotheses is discussed.
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