Downthrown Fault Block Research Articles

Evolution of terminal fans in the Indo-Gangetic foreland basin: A process-response model

The present and paleo-drainage networks, depositional surfaces, fault throw, slope of the downthrown fault blocks, soil properties, sediment characteristics and published optically stimulated luminescence (OSL) ages are used to define the role of tectonics and climate in the growth of forty-five terminal fans (TFs) in the Indo-Gangetic foreland basin. Twenty-three TFs show a single exposed surface throughout, suggesting no or imperceptible breaks in sedimentation during their formation. In contrast, twenty-two TFs show multiple growth surfaces, separated by soil profiles and erosional surfaces. The available optically stimulated luminescence (OSL) dates of these depositional surfaces suggest different phases of TF growth.Tectonically triggered surface faulting altered accommodation space, slope of the substrate, and sediment supply and initiated new TF depositional cycles. The tectonically stable periods, in contrast, did not form TFs, despite the favourable humid climate conditions. Instead, the dry and humid climatic regimes, when combined with frequent surface faulting, produced and remobilized sediments more quickly, promoting TF growth. The humid climate TFs are spatially extensive (average area = 2000 km2), with clay horizons interspersed between sand facies. The dry climates TFs are smaller (average area = 1370 km2), with intervening calcrete horizons and surface salt-efflorescence. Low precipitation and inadequate sediment supply hampered the growth of these dry climate TFs. Despite a considerable period of surface exposure, the surfaces of dry climate TFs lack a well-developed soil profile.The upper Ganga Plain provides more accommodation space across the faults than the middle Ganga Plain. Accommodation space and substrate slope have little effect on TF area unless climatically triggered sediment remobilization occurs. For a given accommodation space and substrate slope, steeper sloping TFs occupy less area than lower sloping ones. Faults with throws of <5 m and substrate slopes of <2° do not form TFs; instead sediment wedges are formed across fault slope breaks.The TFs in the upper Ganga Plain show more growth surfaces than the middle Ganga Plain. The older growth surfaces are either progressively arranged across the direction of channel shift or occur as inliers in between the younger alluvium. Well-developed distributaries on the older TFs disperse sediment more readily, rather than accumulating it in proximal fan regions. However, the younger TFs, with relatively less-developed distributary channel networks, vertically stack sediments in their proximal part and occasionally transport sediment as sheet floods to their distal end regions. The combined geomorphic response of TFs to tectonic events and climatic fluctuations is, therefore, simultaneous for TF growth and stabilization.

PALAEOSEISMOLOGICAL INVESTIGATION OF THE GYRTONI FAULT (THESSALY, CENTRAL GREECE)

Two paleoseismological trenches were excavated across the Gyrtoni Fault in NE Thessaly and studied in order to understand the recent seismotectonic behavior of this structure. Twenty five fluvial-colluvial sediment and pottery samples from both the upthrown and the downthrown fault blocks were investigated. Optically Stimulated Luminescence (OSL) dating has been applied to date both sedimentary depositspalaeosoils and pottery fragments. Paleoseismological analysis of the two trenches indicates evidence of three surface faulting events in the time span between 1.42 ±0.06 ka and 5.59 ± 0.13 ka. The observed vertical displacement per event of ~0.50 m corresponds to an Mw 6.5 ± 0.1 earthquake. An average fault slip rate of 0.41 ± 0.01 mm/yr and an average recurrence of 1.39 ± 0.14 ka for earthquakes were estimated. The results documented the activity of the fault and since the return period from the most recent event (minimum age 1.42 ± 0.06 ka) has expired, the possibility for reactivation of this active structure in the near future should be included in Seismic Hazard Assessment.

APPLICATION OF OSL METHOD IN DATING PAST EARTHQUAKES

The aim of this study was to establish a chronological frame of paleoseismic events of Gyrtoni Fault, (Thessaly, Central Greece), with the use of OSL dating method. The Gyrtoni Fault, defines the north-eastern boundary of the Middle-Late Quaternary Tyrnavos Basin, and was previously investigated with geological methods. Twenty five fluvial-colluvial sediment and pottery samples were collected from two paleoseismological trenches, excavated along the Gyrtoni Fault, from both the upthrown and the downthrown fault blocks. Optically Stimulated Luminescence (OSL) dating was applied to coarse grain quartz using the single-aliquot regenerative-dose (SAR) protocol. Investigations of luminescence characteristics using various tests confirmed the suitability of the material for OSL dating using the SAR protocol. Radioactivity measurements were performed in order to estimate the annual dose rateof the surrounding soils to which the quartz grains were submitted during the burial period of the collected samples. The estimated OSL ages agreed well with the available stratigraphical data, and archaeological evidence. The occurrence of three surface faulting events in a time span between 1.42 ± 0.06 ka and 5.59 ± 0.13 ka was revealed while an earlier faulting event (fourth) was also recognized to be older than 5.59 ± 0.13 ka.

Middle–Late Holocene earthquake history of the Gyrtoni Fault, Central Greece: Insight from optically stimulated luminescence (OSL) dating and paleoseismology

The south-dipping Gyrtoni Fault defines the northeastern boundary of the Middle-Late Quaternary Tyrnavos Basin, Central Greece. The recognition and recent tectonic activity of the fault were previously based on mapping, remote sensing analyses and electrical resistivity tomography studies. To understand the Holocene seismotectonic behavior of the Gyrtoni Fault we excavated two paleoseismological trenches. To estimate the timing of past earthquakes using luminescence dating, we obtained twenty five fluvial-colluvial sediment and pottery samples from both the upthrown and the downthrown fault blocks. We applied the Optically Stimulated Luminescence (OSL) dating to coarse grain quartz using the single-aliquot regenerative-dose (SAR) protocol. Our investigations of luminescence characteristics using various tests confirmed the suitability of the material for OSL dating. We found that the estimated OSL ages were internally consistent and agreed well with the available stratigraphical data, archaeological evidence and radiocarbon dates. The performed paleoseismological analysis emphasized the occurrence of three surface faulting events in a time span between 1.42±0.06ka and 5.59±0.13ka. Also, we recognized an earlier faulting event (fourth) has been also recognized to be older than 5.59±0.13ka. The mean throw per event value of 0.50–0.60m could correspond to a ca. Mw 6.5 earthquake. An average fault slip rate of 0.41±0.01mm/a and an average recurrence time of 1.39±0.14ka were also estimated. Our results suggest that the elapsed time from the most recent event (minimum age 1.42±0.06ka) is comparable with the mean return period.

Three-dimensional structural model of the Cantarell and Sihil structures, Campeche Bay, Mexico

The Cantarell and Sihil fields are located in a complex system of compressive structures in the offshore Campeche Bay province in Mexico. The geometry of the Cantarell–Sihil structure varies along trend from a simple fault-related structure in the south to a duplex consisting of the Cantarell and Sihil structures in the central part to a more complex system consisting of compressive blocks separated by Tertiary normal faults in the north. The Cantarell–Sihil structures formed during three main episodes of deformation: (1) Jurassic to Early Cretaceous extension, resulting in the formation of normal faults that displace Tithonian, Kimmeridgian, and Lower Cretaceous units; (2) Miocene compression, during which the Cantarell–Sihil thrust system was formed; and (3) Pliocene to Holocene extension, during which several of the preexisting Jurassic normal faults were reactivated.The Cantarell field produces out of three separate fault-bounded allochthonous blocks: the Akal, Nohoch, and Kutz blocks. The main field is located in the Akal structure. Secondary fields in the allochthonous block include Kutz, which formed on the crest of a downthrown fault block, and Nohoch, which formed above a west-vergent backthrust. The recently discovered Sihil field is located in a subthrust-compressive structure and consists of two structural lobes formed above the sub-Sihil fault. The Chac structure is formed at the updip edge of a tilted fault block in the autochthonous block. The detailed three-dimensional structural model is being used for future production of the remaining reserves in the Cantarell field as well as for the delineation of the Sihil field.

Geological controls on rock mass classification of coal from Huntly East Mine, New Zealand

Rock Mass Rating (RMR) measurements from 65 sites within Huntly East underground coal mine are presented. All measurements are in coal, for which the dominant discontinuities are vertical cleat. Basic RMR values using two discontinuity spacings are presented: overall RMR based on the average spacing of all individual discontinuities; and cleat zone RMR based on the average spacing between zones of cleat. Cleat orientations are highly variable, but on average approximately parallel horizontal stress axes (face cleat follows maximum horizontal stress axis, butt cleat follows minimum horizontal stress axis). Contours of RMR variations throughout the mine are used to compare rock mass conditions with geological structure. It is apparent that: (1) RMR is least within downthrown fault blocks, and particularly immediately on the downthrown sides of faults, and greatest in upthrown fault blocks; and (2) RMR contours parallel horizontal stress axes within fault-bounded blocks, and bend to parallel faults at block boundaries. From similar contours for parameters contributing to RMR, the Rock Quality Designation (RQD), groundwater rating, and discontinuity condition rating create most of the observed variations in RMR. RQD is determined from the measured discontinuity frequency and hence is a measure of the degree of fracturing of the rock mass. This is interpreted as influencing the groundwater and condition parameters directly by allowing greater water ingress. Discontinuity frequency is greatest (least spacing) in the immediate vicinity of faults, and in downthrown fault blocks, generating low RMR values. Within fault blocks RQD varies little, so RMR contours align with cleat orientations. As RMR contours, faults, stress field and cleat orientation are clearly interrelated, there is unequivocally a connection between RMR and structural geology; this allows some predictive capacity in terms of ground conditions. If geological features can be accurately defined through either drilling programs or seismic surveys, then ground conditions may be predicted before panels are driven.

Influence of strike-slip fault segmentation on drainage evolution and topography. A case study: the Palomares Fault Zone (southeastern Betics, Spain)

The Palomares Fault Zone (PFZ) is one of the main sinistral strike-slip faults in the Betics (SE Spain), with approximately 16 km of north–south displacement. The PFZ initiated during the Tortonian–Messinian as a transfer structure linking areas subject to NW/SE shortening. During the Plio-Quaternary the fault zone lengthened cutting previous fold structures and widened towards the east displacing the active mountain front to the western border of Sierra Almagrera–Sierra Almenara. These Sierras, which show moderate uplift rates of 0.05–0.15 m/ka, formed in response to the oblique-slip regime of the PFZ. The drainage system in the vicinity of the PFZ is asymmetric with respect to a main axial valley that runs parallel to the PFZ on the downthrown fault block. In this block, the drainage density is lower and the streams are longer than in the uplifted block. Furthermore, west of the main axial valley the streams describe eastward directed deflections, indicating the progressive eastward migration of the main axial valley during the Pleistocene. The drainage system on the uplifted ranges shows a consequent pattern, indicating recent uplift and folding under NNW/SSE convergence. Recent activity along segments of the PFZ has increased topographical gradients, favouring dissection of previous streams by headward erosion of streams transverse to the active fault segments.

Reservoir Sequence Analysis: A New Technology for the 90's and Its Application to Oil and Gas Fields: ABSTRACT

Reservoir Sequence Analysis when applied to existing fields can increase the production, life of the field and extend the field with a minimum of cost. In this technology we identify reservoir sands in a standard-of-reference well, to establish a seismic sequence stratigraphic well-tie for the entire field. Age date the Maximum Flooding Surfaces and Sequence Boundaries above and below reservoir sands on a well-log and seismic pro- file and/or workstation using High Resolution Biostratigraphic Analysis, species abundance and diversity histograms and their patterns, and paleoenvironmental paleobathymetric changes. Identify the systems tracts and their corresponding reservoir sands in between age dated Maximum Flooding Surfaces. Interpret the reservoir sands as to type, i.e. IVF, point bar, coastal belt, forced regression, falling stage, bottom-set (shingled) turbidites, slope fan channel, channel overbank, and basin floor fans. Identify and correlate the same individual sands in different wells, and note new sands in a well and sands that shale-out in a well. Correlate the Maximum Flooding Surfaces above and below the reservoir section in additional wells to see which part of the reservoir section and sands have been penetrated. Identify systems tracts in additional wells and construct isopach, sand percent maps of individual systems tract intervalmore » in each well. Correlate sand packages, with a high degree of confidence, from upthrown to downthrown fault blocks, around salt domes, and updip with downdip.« less

Reservoir architecture and diagenesis in downthrown fault block plays: the Lowlander Prospect of Block 14/20b, Witch Ground Graben, Outer Moray Firth, UK North Sea

Abstract The Lowlander Prospect is an Upper Jurassic, downthrown hydrocarbon accumulation, with three-way dip closure and fault seal developed within the Sgiath and Piper formations. The prospect was discovered in 1987 and has since been appraised by more than 900 ft of core from five wells. Detailed core description, conventional core analysis, thin section petrography, fluid inclusion microthermometry, stable isotope analysis and over 2000 ft of wireline petrophysical log interpretation were utilized in constructing an integrated reservoir and diagenetic model for the reservoir units in the prospect. Piper Formation sandstones form the main reservoir in the Lowlander Prospect. Deposition of Piper Formation sandstones was controlled by three major structural elements which served as separate hinterlands; the Fladen Ground Spur and Piper Ridge to the northeast, Halibut Horst and Platform to the southwest, and regional Toarcian-Aalenian domal uplift to the southeast. Two major coarsening up sequences are present in the Lowlander Piper Formation that represent progradational strandline deposits. Transgressive abandonment sandstones, locally preserved beneath a top Piper Formation unconformity, represent a Volgian transgression and associated reworking of Piper Formation sediments. Mechanical compaction and diagenetic cementation, the latter including early non-ferroan calcite, kaolin, quartz overgrowths, ankerite, late calcite and barite-sulphide mineralization, have resulted in generally low intergranular porosities in Piper Formation sandstones. However, total porosities are enhanced by the development of late, post-quartz overgrowth feldspar grain dissolution. Microthermometric study of fluid inclusions in quartz overgrowths indicates that precipitation of authigenic quartz took place at temperatures of between 88 and 122°C, concomitant with hydrocarbon migration. Best reservoir quality is developed adjacent to basin centre faults where early carbonate cements and detrital feldspars have been extensively dissolved. Higher depositional energy facies adjacent to the faults were the initial focal point for aggressive fluid ingress into the reservoir. Fluid migration away from the faults resulted in reprecipitation of carbonate cements up-structure towards the graben edges where reservoir quality is now the lowest. Structural highs thus need not be the prime exploration target in downthrown, hanging-wall basin centre plays.

Use of soils and colluvial deposits in analyzing tectonic events — The southern Arava Rift, Israel

Buried soils in colluvium units developed on downthrown fault blocks have been presumed to form during periods of tectonic quiescence. This assumes however, that all colluvial units on a fault scarp are tectonically induced, particularly for a multiple-event fault scarp. To test this hypothesis we compared the colluvial stratigraphy and pattern of soil development on two non-tectonic scarps (terrace risers) and a nearby multiple-event fault scarp on the same alluvial fan surface. Both slopes developed under extremely and climatic conditions. The colluvial stratigraphy and soil catenary relationship developed on terrace risers were found to be substantially different from those on the multiple-event fault scarp. The fault scarp colluvium contained several colluvial units separated by soils, whereas only two colluvial units without buried soils were found on the terrace risers. The colluvial units on the fault scarp were triangular in shape, while those on the terrace riser were concave and overlapped up the slope. Cumulic soils, indicating a continuous and low rate of deposition during pedogenesis, were formed along the terrace riser. In contrast, buried soils indicating episodic deposition occurred on the downthrown block of the multiple-event fault scarp. Accordingly, the colluvial stratigraphy developed on the fault scarp can be viewed as a record of distinct tectonic events with no climatically controlled deposition. The data indicate that soils and sedimentary stratigraphy can serve as useful indicators to distinguish terrace risers and single-event fault scarps from multiple-event fault scarps. These indicators can be helpful in distinguishing phases of tectonic activity and in evaluating recurrence intervals on a multiple-fault scarp.

Petroleum geology of the Slyne Trough and adjacent basins

Abstract The Slyne Trough lies to the west of Ireland, northeast of the Porcupine Basin and southeast of the margin of the Rockall Trough. The Slyne Trough can be demonstrated to have adequate source, reservoir and seal rocks, but the timing of hydrocarbon migration is more problematical. Structures exist on both large and small scales within the Central and Northern Slyne Troughs, with the possibility of hydrocarbon entrapment at a number of horizons. An excellent source rock has been identified in the Toarcian (Portree Shale Formation) with up to 7% TOC. A secondary source exists within the Sinemurian/Pleinsbachian (Pabba Shale Formation) with up to 4% TOC. Reservoir quality sands have been encountered in the Bathonian (Elgol Sandstone Formation) and Hettangian/Sinemurian (Broadford Beds Formation). A possibility for further reservoir development can be demonstrated in the Bajocian and Pleinsbachian. Sealing intervals have been found in the Bathonian (various levels in the Great Estuarine Group), the Toarcian/Aalenian (Portree Shale Formation/Dun Caan Shale Member) and the Sinemurian/Pleinsbachian (Pabba Shale Formation). The Liassic source rocks are marginally mature in the 27/13-1 well. In the deeper parts of the basin they are probably mature for generation and migration. The Lower and Middle Jurassic evolution of the basin can be estimated with fair accuracy from the 27/13-1 well. The Post-Middle Jurassic history can be interpreted from analogous basins in the region. The North Porcupine sub-basin section shows the complete erosion of the Liassic source horizons. The oil found within the Porcupine basin is believed to be sourced from the Upper Jurassic, which are mature here due to a post-Jurassic history different to that of the Slyne Trough. The Erris Trough is believed to have had a similar Jurassic evolution to the Slyne Trough, but with locally much greater erosion during the Lower Cretaceous. The Jurassic petroleum potential within this basin relies on preservation of the full Liassic sequence in downthrown fault blocks.

Subsurface Models of Coal Occurrence, Oak Grove field, Black Warrior Basin, Alabama

Subsurface investigation of coal occurrence in the Black Creek-Cobb interval of Oak Grove field is based on cross sections and isopach maps made from more than 500 density logs. This study was designed to identify styles of coal occurrence in the Black Warrior basin to aid in coalbed-methane exploration and production. Coal occurrence in parts of the Black Warrior basin may be characterized in terms of end-member fluvial and structural control. Fluvial processes apparently were the major controls on coal occurrence in the Black Creek cycle, where sandstone and coal thickness are inversely related. Additionally, occurrence of thick sandstone sequences above the thickest coal beds suggests that peat compaction provided sites for channel avulsion. In the Mary Lee and Cobb cycles, more coal beds occur in a downthrown fault block than in an upthrown block, and in the Pratt and Cobb cycles, the thickest coal beds occur on the downthrown side of a fault. Only in the Mary Lee cycle, where thick peat accumulated in an abandoned tributary system, is coal thickest on the upthrown block. Most coal beds in Oak Grove field are thickest on the downthrown block because differential subsidence apparently promoted peat accumulation. Clastic influx favors bedsmore » splits in the downthrown block, but joining of beds in the Pratt cycle may reflect sheltering by the fault. In the Mary Lee cycle, in contrast, channel incision evidently provided local relief sufficient for thick peat to accumulate in lows on the upthrown block. Although fluvial and structural processes result in varied styles of coal occurrence, models of coal occurrence have resulted in a predictive framework that may aid in strategic well siting and completion.« less

Effects of Syndepositional Structures on Lower Vicksburg (Oligocene) Fluvial-Deltaic Sedimentation, McAllen Ranch Field Area, Hidalgo County, South Texas: ABSTRACT

The lower Vicksburg sandstones of the McAllen Ranch field area (including McAllen Ranch, Santa Anita, and Guadalupe fields) in south Texas have been significant gas producers for almost 4 decades. However, the relationship of syndepositional structures to the type and distribution of depositional systems is still not fully understood. The importance of growth faults, especially major regional faults, has long been recognized. Further detailed studies concentrating on deeper sandstones of the Vicksburg Formation indicate that these faults in the lower Vicksburg are characterized by low-angle fault-planes and greatly expanded stratigraphic intervals with plunging rollover structures on the downthrown fault blocks. Greatest stratigraphic expansion is associated with channel- outh bar or delta-front facies. Extensive drilling and acquisition of high-resolution seismic data in this area have provided the necessary stratigraphic control to determine the significance of more subtle structural effects on sedimentation. The present study is based on the analysis of 142 well logs, 10 cores, and several seismic lines. The information from cores, characteristic log patterns, and net-sandstone maps of discrete genetic subunits strongly suggests a high-constructive deltaic origin for the reservoir facies. The maps show areas of rapid subsidence associated with large-scale mud movements. These movements result in the formation of sediment-withdrawal basins and fringing syndepositional anticlinal ridges. The localized basins have a profound effect on the style of deltaic sedimentation. Most significant is the exceptional thickening of the distal delta-plain facies characterized by interdistributary bay mud and crevasse splay sands. The splay sands contain abundant macerated plant material. Individual splays have a limited lateral extent, are stacked vertically, and form complex imbricated sand bodies wh ch are gas-prone stratigraphic traps. Such an area presents an ideal target for explorationists and a complex set of problems for the production staffs comtemplating secondary recovery projects. End_of_Article - Last_Page 1431------------

Diagenesis of Middle Miocene Sands in Geopressure Zone of Lirette Field, Terrebonne Parish, Louisiana: ABSTRACT

A study of temperature, pressure, and salinity distribution in the Lirette field reveals important information concerning the effects of the hydrodynamic regime on sandstone diagenesis. Mineralogic examination of associated shales also aids in the understanding of diagenesis in these sands. The Lirette field is a large domal structure related to deep-seated salt, approximately 20,000 ft (6,096 m), bounded to the north and south by major growth faults. Isothermal surfaces in the Lirette field closely follow the structure. Isotherms commonly drop in downthrown fault blocks. Along fault leakage zones, temperatures increase. Pressure distribution in the Lirette field is primarily related to structure, and the presence of a sufficient shale to sand ratio. Formation water salinities are lower (< 50,000 ppm) for wells that have been by geopressured waters. The well-documented decrease of smectite in mixed-layer illite-smectite is present in Lirette shales. A more detailed analysis indicates that some montmorillonite may be converted to beidellite before it is converted to illite. Sandstone diagenesis in the Lirette field is complex and there are significant lateral and vertical variations. The relative sequence of diagenetic events in Lirette sands is as follows: (1) spherulitic calcite cement, probably formed at or near the sediment-water interface; (2) authigenic chlorite rims and platelets, which help to preserve primary porosity; (3) quartz and feldspar overgrowths, uncommon; (4) ferroan calcite cement, due to localized flushing of sandstones by waters released from clay diagenesis; and (6) authigenic kaolinite cement, which reduces porosity along fluid escape routes. Extensive carbonate cement and orthomatrix are the primary contributors to decreased porosity. Late stage kaolinite cement in flushed zones also reduces porosity, but to a lesser extent. End_of_Article - Last_Page 1557------------

Grabens on Anticlines in Gulf Coastal Plain, and Thinning of Sedimentary Section in Downthrown Fault Block: GEOLOGIC NOTES

Grabens are present on many anticlines in the Gulf coastal plain of the southern United States. They are the results of stress fields caused by gravitational forces. It apparently is geometrically impossible for some types of grabens to develop unless there is plastic deformation of sediments within them. Postdepositional thinning of sediments in the downthrown fault block, as observed in model studies and in field examples, seems to allow grabens to develop without creating volume problems. Thinning in the downthrown block is in sharp contrast to the thickening in the downthrown block commonly observed in association with Gulf Coast faults.

Contemporaneous Normal Faults of Gulf Coast and Their Relation to Flexures

Faults along which movement was with sediment deposition are common in the post-Eocene beds of the Gulf Coast. These faults have been called faults, progressive faults, and other names, but the term contemporaneous has priority in geological literature and should be used to describe faults of this type. Increase in the thickness of beds on the downthrown side of a fault when compared with the same beds on the upthrown side is generally considered to be evidence for a fault. In general, the younger the sediments, the more numerous are faults. Faults of this type in the Gulf Coast are largely confined to a belt paralleling the coast line and located downdip from the Vicksburg (Oligocene) flexure. Since movement occurs contemporaneously with deposition, faults control sedimentation in areas affected by the movement. The attitude of the depositional surface of the downthrown fault block largely determines the distribution of sediments over the fault block. A flexure is a zone downdip from which the rate of dip and thickening of sedimentary beds is accentuated. The writers believe that flexures result from deposition of sediments across the shelf break. Flexures affect most Cenozoic beds in the Gulf Coast, and most flexures are marked by faults of regional extent which tend to accentuate the effect of a flexure on sedimentation. Sedimentation rate in the Gulf Coast has been steadily increasing since Cretaceous. Maximum sedimentation rate has risen from 2 cm. per century for Eocene sediments to 8.1 cm. per century for Miocene sediments and 12.2 cm. per century for Pleistocene and Recent sediments. The prevalence of faults in post-Eocene beds is apparently related to this increase in rate of sedimentation and subsidence. Regional faults were formed along zones of flexure as a result of slumping, but faults associated with local structures are the result of normal structure-forming tectonic forces at work in the Gulf Coast during periods of rapid sedimentation.

Ground Water and Geologic Structure of Natchitoches Area, Louisiana

The municipal water supply of the city of Natchitoches has been obtained for many years from shallow wells screened in fine-grained sands of the Wilcox formation. These sands have been over-developed and the supply of water stored in the sands has been depleted gradually as the recharge from rainfall has failed year by year to equal the withdrawals by wells. An exploratory program designed to locate an adequate supply of ground water was initiated in the fall of 1943 by the City of Natchitoches. This program, directed by the Geological Survey, United States Department of the Interior, included the drilling, electric logging, and testing of nine exploratory wells which penetrated water sands of the Sparta sand and of the Wilcox formation. Geological and hydrological data obtained from the test drilling suggest that the city of Natchitoches may be situated on a downthrown fault block defined on the southwest by a fault along Youngs Bayou and on the southeast by a fault extending from Old River toward Cane River Lake. Salt water, unlike connate sea water in composition, is present in the sands underlying most of the city. A similarity of topographic, geologic, and hydrologic conditions in this area to those in salt-dome areas in near-by parishes is evident. Southwest of the city, abundant supplies of soft water were found in the Sparta sand and in the sands of the Wilcox formation. These sands crop out in a narrow band extending from Natchitoches into Texas, and generally yield moderately large quantities of potable water in the areas immediately south of their outcrops. A supply of at least 1,000,000 gallons a day for the city of Natchitoches may be obtained from three to five properly developed supply wells in Secs. 10, 56, and 57, T. 8 N., R. 7 W. Additional water for increased needs of the future may be provided by extending this well field toward the west.