Karst Hydrogeology and geomorphology

A lecture heavily inspired by Ford and Williams' Karst Hydrogeology and geomorphology, second edition, presented at a caver audience at Imperial college.

Transcript

1. Karst hydrogeology and geomorphology An overview
2. Overview ¤  Session 1 ¤  Introduction to karst ¤  The karst rocks ¤  Lithological factors affecting karst development ¤  Session 2 ¤  Hydrological concepts ¤  Karst aquifer analysis ¤  Speleogenesis
3. Introduction to Karst
4. What is ‘Karst’? A karstified terrain has distinctive hydrology and landforms that arise from a combination of high rock solubility and well developed secondary fracture porosity
5. Characteristics ¤  Sinking streams ¤  Caves ¤  Enclosed depressions – macro-scale! ¤  Fluted rock outcrops – meso-scale ¤  Large springs
6. Characteristics
7. Further nuances Exokarst vs Endokarst ¤  Suite of karst features developed on the surface as opposed to undeground Karst vs pseudokarst ¤  Karstic features formed by dissolution process as opposed to a phase change (e.g. glacier caves)
8. Exotic karsts Vulcanokarst Thermokarst
9. Global distribution of karst Location of the major carbonate rock outcrops – highly simplified
10. The ‘comprehensive karst system’ ¤  A conceptual model tying together all phenomena operating in different karst terrains ¤  Two major zones distinguished: ¤  Primarily erosional zone – where most caves tend to form ¤  Primarily depositional zone –where the carbonate rocks are formed
11. The ‘comprehensive karst system’ Geomorphology deals with the study of landforms and processes behind them
12. The karst rocks
13. The karst rocks Karst rocks are in general more soluble than other substrates
14. What are the main groups? ¤  Carbonates ¤  Limestone ¤  Dolomite ¤  Marble ¤  Silica-rich ¤  Quartzite sandstones ¤  Evaporites ¤  Gypsum ¤  Halite
15. The carbonate rocks •  Need more than >50% carbonate minerals •  Pure end-members are calcite/aragonite and dolomite (mineral)
16. Why are carbonate rocks different? ¤  Rates of deposition depend on organic productivity ¤  They are more prone to post-depositional alteration than any other sediment ¤  Big field of study is ‘diagenesis’ or second life of carbonate rocks as they are buried ¤  Long history of recrystallisation alters permeability, porosity, aquifer quality etc…
17. Why does the depositional environment matter? ¤  Limestones are the most significant contributor to karst rocks ¤  The environment of deposition dictates much of: ¤  Bed thickness ¤  Rock purity ¤  Texture ¤  While limestones form almost everywhere today, only tropical shelves and ramps go on to form karst domains
18. Building blocks of carbonate rocks Micrite or carbonate muds and carbonate sands (shell fragments, faecal pellets, etc…)
19. Building blocks of carbonate rocks Reef frameworks
20. Terrestrial carbonates Tufa (accreted on organic matter) Travertine (crystalline and inorganic
21. So we classify them There are two schools: Dunham’s and Folk’s. The first focuses on mud content, the second on ‘allochems’. Above is Folk’s classification.
22. Linking ‘rock type’ to depositional environments ¤  Notion of rock facies ¤  A facies is a body of rock with specified characteristics, ¤  They can be any observable attribute: ¤  overall appearance, ¤  composition, ¤  condition of formation,
23. The carbonate depositional environments (NASA 2003)
25. Non-carbonates Evaporites Quartzites
26. Lithological factors affecting karst
27. Lithological factors affecting karst development ¤  Rock purity /interbedded ‘clastic’ rocks ¤  Grain size and texture ¤  Porosity ¤  Rock strength ¤  Fractures and joints ¤  Rock deformation
28. Lithological factors affecting karst development Rock purity: Clay minerals and silica are the most common insoluble impu- rities in carbonate rocks. Interbedded clastics: Bedding plane thickness:
29. Porosity Mudstone to wackestone: no primary porosity Oolitic packstone with interclast porosity >30%
30. Rock strength
31. Fractures and joints
32. Faults Several kms of displacement Large damage zones
33. Rock deformation Extensional brittle deformation confined at crest of anticlines Bed slip in limbs of folds Folding carbonate/silicate mix creates conditions of artesian confinement
34. End of session 1 Refreshments!
35. Hydrological concepts
36. Hydrogeological concepts Aquifers store and transmit water Aquicludes cannot transmit vast quantities when saturated
37. Water table and flow nets Water in an unconfined aquifer descends freely under gravity until it finds its own level, known as the water table
38. Total porosity vs Effective porosity Refers to those pores which are hydrologically connected
39. Three porosities Granular Fracture Conduit
40. Flow through granular porous media Discharge can be calculated using the Hagen Poiseulle formula Discharge is proportional to hydraulic head drop, to 4th power of pore radius. The Hagen-Poiseulle law: laminar Flow through a tube Darcy’s law: laminar flow through Porous mdia
41. Input control Two types: Allogenic (foreign) and Autogenic (precipitation falls directly on karst.
42. Input control: allogenic recharge This is scenario A in the Yorkshire Dales. The Yoredale group lies over Great Scar Limestone
43. Output control
44. Zones of a karst aquifer
45. Speleogenesis
46. Classification of cave systems
48. Conduit propagation with single input Simplest case scenario: Propagation in direction of greatest hydraulic gradient Notion of ‘victor tube’.
49. Numerical modelling It considers a single input into a bedding plane at a depth of 2 m. Flow from the plane into the matrix and out again is incorporated in a three-dimensional mesh 500 m in length, 100 m wide and 3 m deep, that incorporates >400 000 nodes. Breakthrough is achieved in about 15 000 year.
50. Single rank – multiple inputs
51. Multirank input Cave systems are connected due to headward recession of piezometric surface
52. The Four state model Relative sizes of vadose and phreatic zones dependent on bedding plane±fissure frequency (fault, joints etc…)
53. A different take on alpine systems These are proposed after most alpine caves (Palmer and Audra 2004).
54. Multiphase cave systems
55. Passage morphologies In the phreatic zone
56. Passage morphologies In the phreatic zone
57. Passage morphologies In the phreatic zone
58. Passage morphologies In the phreatic zone
59. Smaller features in phreatic passages Solution pockets Pendants
60. Passage morphologies In the vadose zone: shaft and canyon development
61. Passage morphologies ‘Potholing’ and evolution of stream potholes
62. Passage morphology Idea of vadose entrenchment
63. Passage morphologies In the vadose zone: shaft and canyon development
64. Passage morphologies In the vadose zone: shaft and canyon development
65. Passage morphologies Wall scalloping
66. Passage morphologies Wall scalloping
67. Breakdown features Roof sag and failure in tension of the dome
68. Breakdown features
69. End of Session 2 Thanks for listening!