Influence of Mechanical Layering and Natural Fractures on Undercutting and Rapid Headward Erosion (Recession) at Canyon Lake Spillway, Texas, U.S.A

This study investigates the role of mechanical layering and fractures on flood-related erosional undercutting and resulting rapid spillway recession. In the summer of 2002, 86 cm of rain fell in an 8-day period across the Guadalupe River drainage basin in central Texas, causing Canyon Lake reservoir to completely fill and overtop the emergency spillway for the first time. The resulting flood incised a gorge into the mechanically layered Glen Rose Formation and caused headward erosion (recession) at the downstream edge of the emergency spillway. Comparison of pre- and post-flood imagery and assessment of flood records indicates that maximum recession localized at the northern end of the emergency spillway where 28 m recession occurred. This recession occurred at an estimated rate of up to 10 m/day during the first ~3 days of the flood, which is among the highest rates of recorded bedrock recession. Analysis of historical photographs, field observations and measurement of erosional undercutting, along with measurements of fracture orientation, fracture spacing, and mechanical rebound are used to understand rock mass characteristics that influenced erosional undercutting and rapid recession of the spillway. Evidence of significant undercutting was observed where incompetent argillaceous wackestone (marl) underlies competent limestone. These results reveal that the greatest amount and rate of recession of the spillway was associated with undercutting and toppling collapse of fracture-bounded limestone blocks. Block size may be a factor in continuation of the process, in that large blocks may accumulate at the base of the scarp and inhibit continued erosional undercutting, whereas in other areas smaller eroded blocks can be carried away by the floodwaters and undercutting may continue, facilitating recession. The combination of mechanical contrast between layers and natural fractures in competent layers together contributed to exceptionally high rates of headward erosion. Observed rock mass erodibility behavior was in the range of medium to high erodibility in limestone with widely spaced fractures that would normally be expected to have very low erodibility. Bulk rock mass erodibility in this situation was similar to the most erodibile layer, specifically, the marl at base of spillway pour-off cliff

Fault zone deformation controlled by carbonate mechanical stratigraphy, Balcones fault system, Texas

Normal faults in Cretaceous carbonates in the Balcones fault system provide important analogs for fault zone architecture and deformation in carbonate reservoirs worldwide. Mechanical layering is a fundamental control on carbonate fault zones. Relatively planar faults with low-displacement gradients develop in massive, strong, clay-poor limestones and dolomites. In less competent clay-rich strata, shale beds impede fault propagation, resulting in fault-related folding, and locally steep bedding dips. Faults in clay-poor massive limestones and dolomites tend to be steep (70deg or more), whereas weaker, clay-rich limestones develop faults with shallower dips (60deg or less). Fault zone rocks show evidence of cataclasis, cementation, deformation of cement by mechanical twinning and pressure solution, and multiple generations of cement with differing degrees of deformation, indicating contemporaneous cementation and fault slip. In stratigraphic sequences consisting of both competent and incompetent strata, the ratio of incompetent to competent strata by thickness is a useful guide for inferring the relative rates of fault displacement and propagation. Low displacement-to-propagation ratios associated with competent strata generate low-displacement gradients, inhibiting fault-related folding. Conversely, high displacement-to-propagation ratios associated with incompetent strata promote high-displacement gradients and fault-related folding

Discrete element modeling of extensional fault-related monocline formation

The interplay of faulting and folding has long been recognized in extensional systems and numerous investigations have documented the importance of mechanical layering on development of normal-fault related monoclines. In this study we use discrete element modeling to explore the impact of mechanical layering and fault geometry on normal-fault related folding for a well-exposed field example in southwestern Iceland. The model honors the mechanically stratified character of the deformed sequence and replicates the monocline geometry by simulating displacement and upward propagation of a buried fault. A close match between model geometry and field observations was obtained using a refracted fault geometry and a cover sequence with alternating 40-m-thick relatively strong and 10-m-thick relatively weak layers. Initial deformation is accommodated by folding of mechanically weaker intervals and fracturing of mechanically stronger layers, and maximum monocline width is developed early. As overall fault displacement increases, throughgoing fracture connections along the footwall side of the monocline develop, leaving the monocline limb attached to the hanging wall. Consistent with field observations, model results suggest that significant near-surface and subsurface fracture porosity is developed in strong layers. Reproducing the complexity of the natural normal fault-related monocline in Iceland requires incorporating strongly contrasting mechanical properties within a layered cover sequence

Introduction to the special issue on new dynamics in palaeostress analysis [Editorial]

This Special Issue contains papers presented at the New Dynamics in Palaeostress conference held under the auspices of the Tectonic Studies Group of the Geological Society of London in Burlington House on 10th September 2004. The impetus for organising this conference was the rapid developments in the subject that have occurred in the last few years; it is clearly undergoing a renaissance since the formative developments some three decades ago

Geologic structure of the Edwards (Balcones Fault Zone) Aquifer

The Edwards (Balcones Fault Zone) Aquifer is structurally controlled by the system of normal faults following the Balcones Escarpment, with major domains, including contributing, recharge (unconfined), and artesian (confined) zones, dictated by the large-displacement (50 m to \u3e250 m throw) normal faults and depth of erosion. Faults and extension fractures, in many cases enhanced by dissolution, localize recharge and flow within the Balcones fault zone and into the subsurface of the artesian zone. Juxtaposition of the Edwards with other aquifers provides avenues for interaquifer communication, while juxtaposition against impermeable layers and concentration of clay and mineralization along faults locally produce fault seals for compartmentalization and confinement. Fault block deformation, including small faults and extension fractures, leads to aquifer permeability anisotropy. Faults also locally provide natural pathways for groundwater discharge through springs above the confined (artesian) zone. Although the importance of joints and faults in the Edwards (Balcones Fault Zone) Aquifer system is recognized, there has not been a systematic analysis of the meter-scale structures in the Edwards and associated confining units and their influence on groundwater flow. Here, we review evidence from several key areas showing that an analysis of faults and fractures in the Edwards (Balcones Fault Zone) Aquifer and associated aquifers and confining units is needed to characterize structural fabrics and assess the permeability architecture critical for the next generation of groundwater modeling of the aquifer

Fault Zone Deformation and Displacement Partitioning in Mechanically Layered Carbonates: The Hidden Valley Fault, Central Texas

The Hidden Valley fault is exposed in Canyon Lake Gorge (central Texas) and cuts the Cretaceous Glen Rose Formation. This exposure provides an opportunity to explore the relationship between deformation mechanisms and fault displacement along 830 m (2723 ft) of a normal fault typical of those in carbonate reservoirs and aquifers around the world. The fault zone has five domains: gently deformed footwall damage zone, intensely deformed footwall damage zone, fault core, intensely deformed hanging-wall damage zone, and gently deformed hanging-wall damage zone. Footwall deformation is more intense and laterally extensive than hanging-wall deformation, and the intensely deformed hanging-wall damage zone is narrow and locally absent. The fault core contains thin clay-rich gouge or smear in most places but is locally represented by only a slickensided surface between limestone layers. The 55- to 63-m (180–207-ft) fault throw across a 43- to 98-m (141- to 322-ft)-wide fault zone is accommodated by slip along the fault core, layer tilting (synthetic dip development) in footwall and hanging-wall damage zones, and distributed faulting in footwall and hanging-wall damage zones. Total offset across the fault overestimates actual stratigraphic offset by 8 to 12 m (26–39 ft) or about 14 to 21%. In our interpretation, the Hidden Valley fault zone records both early extensional folding of the Glen Rose Formation and subsequent normal faulting that propagated downward from the overlying competent Edwards Group. The damage zone width is thus established before fault breakthrough

Geologic Controls on Interaction Between the Edwards and Trinity Aquifers, Balcones Fault System, Texas

Faults of the Balcones fault system exert important controls on the groundwater hydrology of the Edwards and Trinity Aquifers, including the following: (i) faults juxtapose permeable and relatively impermeable hydrogeologic units, (ii) the normal fault system causes structural thinning of the Edwards and Trinity Aquifer strata, and (iii) faults provide potential pathways for infiltration of water into the groundwater systems and for lateral and vertical movement of groundwater. We present examples of these structural geologic controls on aquifer properties using data and observations from the Helotes 7.5-minute quadrangle and the Hidden Valley fault exposed in Canyon Lake Gorge. Geologic framework modeling of the Helotes quadrangle illustrates the strong potential for direct communication between the Edwards Group and Glen Rose Formation in this area. The 100+ m displacement of the Haby Crossing fault is responsible for dropping the Edwards Aquifer from hilltop exposures north of the fault to mostly buried (confined) on the south side of the fault. Consequently, the area designated as Edwards Aquifer recharge zone is at its narrowest in this part of the Balcones Fault Zone. The Hidden Valley fault has an estimated 60-70 m of throw (vertical component of displacement) along the approximately 800 m of Upper Glen Rose Limestone exposure at Canyon Lake Gorge. Water ponds on the fault zone in some places, sinks into the fault along other stretches, and discharges laterally from the fault zone in yet another. These examples of locally high permeability of an exhumed fault show the importance of map-scale faults for groundwater flow. They also cast doubt on interpretations that such faults would act as barriers to across-fault flow in cases where such faults juxtapose Glen Rose Limestone with Glen Rose Limestone or the more permeable Edwards Limestone with Glen Rose Limestone. High permeability zones under and near stream channels may serve as fast pathways of communication from the Trinity to Edwards Aquifers. Direct

Geologic Controls on Interaction Between the Edwards and Trinity Aquifers, Balcones Fault System, Texas

Faults of the Balcones fault system exert important controls on the groundwater hydrology of the Edwards and Trinity Aquifers, including the following: (i) faults juxtapose permeable and relatively impermeable hydrogeologic units, (ii) the normal fault system causes structural thinning of the Edwards and Trinity Aquifer strata, and (iii) faults provide potential pathways for infiltration of water into the groundwater systems and for lateral and vertical movement of groundwater. We present examples of these structural geologic controls on aquifer properties using data and observations from the Helotes 7.5-minute quadrangle and the Hidden Valley fault exposed in Canyon Lake Gorge. Geologic framework modeling of the Helotes quadrangle illustrates the strong potential for direct communication between the Edwards Group and Glen Rose Formation in this area. The 100+ m displacement of the Haby Crossing fault is responsible for dropping the Edwards Aquifer from hilltop exposures north of the fault to mostly buried (confined) on the south side of the fault. Consequently, the area designated as Edwards Aquifer recharge zone is at its narrowest in this part of the Balcones Fault Zone. The Hidden Valley fault has an estimated 60-70 m of throw (vertical component of displacement) along the approximately 800 m of Upper Glen Rose Limestone exposure at Canyon Lake Gorge. Water ponds on the fault zone in some places, sinks into the fault along other stretches, and discharges laterally from the fault zone in yet another. These examples of locally high permeability of an exhumed fault show the importance of map-scale faults for groundwater flow. They also cast doubt on interpretations that such faults would act as barriers to across-fault flow in cases where such faults juxtapose Glen Rose Limestone with Glen Rose Limestone or the more permeable Edwards Limestone with Glen Rose Limestone. High permeability zones under and near stream channels may serve as fast pathways of communication from the Trinity to Edwards Aquifers. Direct