Central Texas’ Llano Uplift: The Keystone for Recon-structing the Mesoproterozoic through Cambrian History of Southern Laurentia

Research Article| February 01, 2000 Mesoproterozoic chronostratigraphy of the southeastern Llano uplift, central Texas Joseph F. Reese; Joseph F. Reese 1Department of Geological Sciences, University of Texas, Austin, Texas 78712 Search for other works by this author on: GSW Google Scholar Sharon Mosher; Sharon Mosher 1Department of Geological Sciences, University of Texas, Austin, Texas 78712 Search for other works by this author on: GSW Google Scholar James Connelly; James Connelly 1Department of Geological Sciences, University of Texas, Austin, Texas 78712 Search for other works by this author on: GSW Google Scholar Robert Roback Robert Roback 1Department of Geological Sciences, University of Texas, Austin, Texas 78712 Search for other works by this author on: GSW Google Scholar GSA Bulletin (2000) 112 (2): 278–291. https://doi.org/10.1130/0016-7606(2000)112<278:MCOTSL>2.0.CO;2 Article history received: 30 Jun 1997 rev-recd: 19 Feb 1999 accepted: 11 Mar 1999 first online: 01 Jun 2017 Cite View This Citation Add to Citation Manager Share Icon Share MailTo Twitter LinkedIn Tools Icon Tools Get Permissions Search Site Citation Joseph F. Reese, Sharon Mosher, James Connelly, Robert Roback; Mesoproterozoic chronostratigraphy of the southeastern Llano uplift, central Texas. GSA Bulletin 2000;; 112 (2): 278–291. doi: https://doi.org/10.1130/0016-7606(2000)112<278:MCOTSL>2.0.CO;2 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyGSA Bulletin Search Advanced Search Abstract The Llano uplift of central Texas exposes Mesoproterozoic crystalline rocks affected by a Grenvillian orogeny between 1.2 and 1.1 Ga. We report four new U-Pb zircon protolith ages of 1247 ± 4, 1257 ± 3, 1272 +8/−5, and 1366 ± 3 Ma for four quartzofeldspathic rocks from the regionally defined Packsaddle Schist and Valley Spring Gneiss in the southeastern part of the uplift. The 1366 ± 3 Ma gneiss also yields a metamorphic age of 1325 ± 5 Ma. These new U-Pb ages, in conjunction with previous U-Pb age data, demonstrate that several regionally defined map units contain constituents with widely disparate ages.We have identified four age suites for protoliths of metamorphic rocks in the southeastern part of the Llano uplift that appear to represent rock packages of distinct tectonic origins: (1) the northern, 1366–1272 Ma felsic gneisses of volcanic, plutonic, and continentally derived sedimentary origin (Valley Spring Gneiss and an older gneiss component), (2) the geographically intermediate, ca. 1257–1247 Ma (and possibly older) basinal sequence that formed along a continental shelf and slope near an arc (Packsaddle Schist), (3) the southern, 1326–1275 Ma remnant of an exotic, ensimatic arc complex (Big Branch Gneiss and Coal Creek Serpentinite and plutonic complex), and (4) the 1239–1232 Ma tectonized felsic rocks that intruded the first two suites. The first three suites of rocks were structurally imbricated during Grenville orogenesis, and the fourth suite may either represent early synorogenic crustal melts or be related to the volcanic rocks in the Packsaddle Schist.In addition, these new data show that, contrary to previous reports, younger Packsaddle Schist lies in structural contact above older components of the Valley Spring Gneiss. Moreover, one of the dated meta-igneous units is the oldest unit yet found in the uplift and records an early period of metamorphism prior to formation of most Llano uplift rocks. Its 1366 ± 3 Ma protolith age is coeval with rocks of the Western Granite-Rhyolite terrane. Age and geochemical similarities link this unit of unknown regional extent with the Western Granite-Rhyolite terrane of known Laurentian affinity. Thus, this component of the Llano uplift may be the basement on which the younger components of the Valley Spring Gneiss and Packsaddle Schist formed in a tectonic setting proximal to Laurentia and south of the Western Granite-Rhyolite terrane. You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

The Llano Uplift in central Texas, USA, exposes the southernmost expanse of Laurentian crystalline basement in North America and the overlying lower Paleozoic strata deposited on the Great Unconformity. Systematic detrital zircon (DZ) U-Pb provenance analysis of the Hickory Sandstone—the basal unit of the Cambrian Riley Formation that onlaps the Mesoproterozoic core of the Llano Uplift—yielded locally variable DZ U-Pb signatures with ages ranging from 1800 Ma to 485 Ma (n &amp;gt;1700). The Hickory Sandstone zircons are dominated by 1550−1300 Ma (50%) and 1300−1000 Ma (46%) ages. These two dominant Mesoproterozoic DZ age components likely were sourced from local Grenville metamorphic and igneous basement of the Llano Province (1300−1000 Ma) and from the extra-regional Granite-Rhyolite Province basement to the north and northwest of the Grenville Front, which suggests both local sourcing and a regional drainage system supplying sediment to the southern Laurentian margin during the Cambrian. This interpretation is supported by a minor component of 1800−1600 Ma zircons, sourced from the Yavapai-Mazatzal Province, and zircons with Early Cambrian ages that were likely sourced from the Wichita Igneous Province, which is located to the northwest and to the north of the Llano Uplift, respectively. Documented NNW-trending topographic ridges in the Mesoproterozoic basement surface of the Llano Uplift could have funneled aeolian and fluvial sand supply to the southern Laurentian coast that was reworked in the marginal marine environment. Samples from the western Llano Uplift are dominated by regionally sourced, early Mesoproterozoic Granite-Rhyolite Province DZ ages, whereas samples from the eastern Llano Uplift exhibit a dominant locally sourced Grenville signature. Hickory Sandstone samples also contain a small number of Neoproterozoic (850−600 Ma) and Cambrian (541−487 Ma) zircons. Sources for Neoproterozoic zircons likely were located along the Laurentian continental margins and derived from extension-related magmatism associated with the breakup of Rodinia. Cambrian zircons are most common in Hickory Sandstone samples on the southern and western flanks of the Llano Uplift. Some are reasonably sourced from the Wichita Igneous Province to the north, but the younger Cambrian zircons suggest sources to the west. Upper Hickory strata are dominated (&amp;gt;60%) by 1550−1300 Ma grains with up to 10% &amp;gt;1600 Ma Paleoproterozoic grains. Upper Cambrian sandstones of the Wilberns Formation also contain a significant contribution of &amp;gt;1600 Ma grains, which suggests a possible enlargement of the fluvial headwaters over time extending into older Laurentian provinces to the north and northwest. Differences among DZ populations over relatively limited distances may have been related to NW-oriented ridge and corridor landforms that developed on the Great Unconformity surface, which were products of the Proterozoic tectonic and lithologic architecture affected by aeolian and fluvial processes at this unusual stage in Earth history. Paleocurrent data and atypical detrital grain characteristics for a Hickory Sandstone locality in the southeastern Llano Uplift and its DZ age similarities with Cambrian sandstones of the Argentine Precordillera suggest a common provenance prior to or during Rodinia breakup.

Research Article| January 01, 2008 Mesoproterozoic plate tectonics: A collisional model for the Grenville-aged orogenic belt in the Llano uplift, central Texas S. Mosher; S. Mosher 1Department of Geological Sciences, Jackson School of Geosciences, University of Texas at Austin, Austin, Texas 78712, USA Search for other works by this author on: GSW Google Scholar J.S.F. Levine; J.S.F. Levine 1Department of Geological Sciences, Jackson School of Geosciences, University of Texas at Austin, Austin, Texas 78712, USA Search for other works by this author on: GSW Google Scholar W.D. Carlson W.D. Carlson 1Department of Geological Sciences, Jackson School of Geosciences, University of Texas at Austin, Austin, Texas 78712, USA Search for other works by this author on: GSW Google Scholar Geology (2008) 36 (1): 55–58. https://doi.org/10.1130/G24049A.1 Article history received: 01 May 2007 rev-recd: 05 Sep 2007 accepted: 06 Sep 2007 first online: 09 Mar 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation S. Mosher, J.S.F. Levine, W.D. Carlson; Mesoproterozoic plate tectonics: A collisional model for the Grenville-aged orogenic belt in the Llano uplift, central Texas. Geology 2008;; 36 (1): 55–58. doi: https://doi.org/10.1130/G24049A.1 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyGeology Search Advanced Search Abstract The Llano uplift of central Texas, United States, exposes the core of a Mesoproterozoic orogenic belt that formed along the southern margin of Laurentia during Grenville time. A new collisional model is proposed that reconciles differences in structural stacking, apparent tectonic transport, and deformation conditions between the eastern and western portions of the uplift and explains uplift and exhumation of high-pressure eclogitic rocks, emplacement of ophiolitic rocks, and subsequent late-stage to postcollisional plutonism. Our model proposes that subduction with southward polarity resulted in collision of an exotic arc with Laurentia, emplacement of ophiolitic rocks, and telescoping of the intervening basinal sediments, followed by overriding of the arc and margin of Laurentia by a southern continent with transport toward Laurentia. The model further proposes that convergence led to subduction of the Laurentian margin, resulting in high-pressure metamorphism, but buoyancy forces due to subduction of continental crust under the southern continent resulted in uplift and retrotransport away from Laurentia, in a manner similar to that proposed for the Alpine orogeny. Slab breakoff resulted in upwelling of the asthenosphere, leading to intrusion of juvenile granitic plutons. Subduction along strike caused continued contraction that waned with time. The eastern uplift records continent-arc-continent collision, whereas the western uplift records continent-continent collision; the two regions also expose different crustal levels in the orogen. The striking similarity with Phanerozoic orogens, including emplacement of ophiolites and formation of high-pressure rocks, implies that plate tectonic processes including subduction were active prior to the Neoproterozoic. You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

A clearer concept of the growth of the Llano uplift and related principal structural features of central Texas is sought through use of the present increased store of surface and subsurface geological data. Generalized thickness maps have been prepared as an aid to analysis of time, place, and amount of major uplifts and subsidences, some of which were intense, localized tectonic movements, others widespread epeirogenic adjustments. The most obvious major tectonic feature in central Texas is the Llano uplift where the pre-Cambrian basement complex crops out in an area 40 miles wide and 70 miles long. During much of early Paleozoic time this area was a seaway and for long intervals thereafter it was alternately above and below sea-level. Major diastrophic changes occurred after Early Ordovician and before Late Pennsylvanian, by which latter time the pre-Cambrian surface had become at least 10,000 feet higher in the western part of the Llano uplift than beneath the flanking Fort Worth and Kerr basins. Erosional losses indicate that uplift accounted for about one-third and subsidence two-thirds of the vertical movement in these structural adjustments. Many pronounced structural features observed in the Llano uplift show a surprising lack of parallelism with the main northwest axial trend. Instead, a north to north-northeast trend is common, thereby dividing this uplift into several major segments which are bounded by relatively steep dips and extensive normal faults with vertical displacement as great as 3,000 feet. These major intersecting tectonic features appreciably affected only those beds that are older than the Lazy Bend group (restricted) of the Strawn series. Thereafter structural trends developed mainly along northwest trends. Mississippian outcrops in the Llano region transgress the truncated Ordovician Ellenburger group. Drilling has shown an increasing loss of section west of the Llano uplift so that, as a result of both erosion and non-deposition, Upper Pennsylvanian (Canyon) marine sediments locally overlap Cambrian rocks in and near northeast Menard County. Farther west and northwest, Middle Pennsylvanian beds rest on truncated Mississippian and Ordovician or older rocks in a large region, heretofore called the Concho arch, where local as well as regional tectonic features had developed mainly along trends varying from north-northeast to northwest. Thin Middle Pennsylvanian marine sediments of the Lampasas and Strawn series deposited across this base-levelled region are chiefly limestones and shales o the platform type in contrast to thick basinal type deposits on the east and south. The northwest part of the Llano area evidently remained somewhat above sea-level during most or all of the Strawn time. However, at least 2,000 feet of Upper Pennsylvanian and Lower Permian sediments are assumed to have been deposited across the Llano region, judged by their thickness and marine character, also by projection of regional dip, in near-by outcrops. Regional upward tilting toward the east, followed by extensive erosion, has brought prominence to the Llano uplift. As disclosed by drilling, a regional downward tilting toward the west has given the eastern part of the Concho platform the appearance of a major arch, commonly referred to as the Bend arch or flexure. The much larger Concho arch and broad Concho platform between the Bend axis and the Midland basin lost much of their original dominant position as a result of this regional tilting which began in Late Pennsylvanian, but took place mainly during Permian time. Subsequently, epeirogenic movements, including continental emergence after the Lower Cretaceous epoch, have permitted erosion of all sediments and re-exposure of pre-Cambrian basement in the Llano area, now known geogr phically as the Central Mineral region.

&lt;p&gt;Table S1: Detrital zircon sample descriptions and locations from outcrops, roadcuts, and quarries around the Llano Uplift, central Texas. Table S2: Detrital zircon U-Pb geochronologic analyses of central Texas Cambrian sandstones. Figure S1: Distribution of Cambrian sandstones samples in the Llano Uplift with zircons of various age groups.&lt;/p&gt;

&lt;p&gt;Table S1: Detrital zircon sample descriptions and locations from outcrops, roadcuts, and quarries around the Llano Uplift, central Texas. Table S2: Detrital zircon U-Pb geochronologic analyses of central Texas Cambrian sandstones. Figure S1: Distribution of Cambrian sandstones samples in the Llano Uplift with zircons of various age groups.&lt;/p&gt;

Regional Stratigraphy of Mid-Continent

The Llano uplift of central Texas exposes Mesoproterozoic crystalline rocks affected by a Grenvillian orogeny between 1.2 and 1.1 Ga. We report four new U-Pb zircon protolith ages of 1247 ± 4, 1257 ± 3, 1272 +8 / −5 , and 1366 ± 3 Ma for four quartzofeldspathic rocks from the regionally defined Packsaddle Schist and Valley Spring Gneiss in the southeastern part of the uplift. The 1366 ± 3 Ma gneiss also yields a metamorphic age of 1325 ± 5 Ma. These new U-Pb ages, in conjunction with previous U-Pb age data, demonstrate that several regionally defined map units contain constituents with widely disparate ages. We have identified four age suites for protoliths of metamorphic rocks in the southeastern part of the Llano uplift that appear to represent rock packages of distinct tectonic origins: (1) the northern, 1366–1272 Ma felsic gneisses of volcanic, plutonic, and continentally derived sedimentary origin (Valley Spring Gneiss and an older gneiss component), (2) the geographically intermediate, ca. 1257–1247 Ma (and possibly older) basinal sequence that formed along a continental shelf and slope near an arc (Packsaddle Schist), (3) the southern, 1326–1275 Ma remnant of an exotic, ensimatic arc complex (Big Branch Gneiss and Coal Creek Serpentinite and plutonic complex), and (4) the 1239–1232 Ma tectonized felsic rocks that intruded the first two suites. The first three suites of rocks were structurally imbricated during Grenville orogenesis, and the fourth suite may either represent early synorogenic crustal melts or be related to the volcanic rocks in the Packsaddle Schist. In addition, these new data show that, contrary to previous reports, younger Packsaddle Schist lies in structural contact above older components of the Valley Spring Gneiss. Moreover, one of the dated meta-igneous units is the oldest unit yet found in the uplift and records an early period of metamorphism prior to formation of most Llano uplift rocks. Its 1366 ± 3 Ma protolith age is coeval with rocks of the Western Granite-Rhyolite terrane. Age and geochemical similarities link this unit of unknown regional extent with the Western Granite-Rhyolite terrane of known Laurentian affinity. Thus, this component of the Llano uplift may be the basement on which the younger components of the Valley Spring Gneiss and Packsaddle Schist formed in a tectonic setting proximal to Laurentia and south of the Western Granite-Rhyolite terrane.

Late thermal evolution of Proterozoic rocks in the northeastern Llano Uplift, central Texas

Zircon U–Pb ages and Hf isotopic compositions indicate multiple sources for Grenvillian detrital zircon deposited in western Laurentia

The Cretaceous-preCretaceous unconformity, i. e., base Zuni supersequence boundary, crops out across much of central and west Texas. Topographic features on the unconformity surface are exhumed along the erosional out-crop edge of the nearly flat-lying Cretaceous strata that overlie the surface within Lampasas and Mills counties, Texas. Digital elevation data displayed as a topographic profile atop Pennsylvanian outcrops adjacent to the Cretaceous outcrop edge indicate that paleotopography was influenced by northwest-dipping Pennsylvanian strata that underlie the unconformity surface. Differential erosion of Pennsylvanian limestone, sandstone, and shale prior to Cretaceous deposition created a trellis network of northeast-trending valleys and divides. Local tectonic features, climate, and rising sea level influenced the nature of the Cretaceous fluvial fill that directly overlies the unconformity surface. The Sycamore Conglomerate is the earliest Cretaceous deposit, and accumulated in a sub-humid or drier climate as a downfilling braided alluvial wedge shed off the Llano Uplift. The Travis Peak Formation, a conglomeratic sandstone, onlaps and disconformably overlies the Sycamore Conglomerate, and was deposited as an aggradational meandering river complex during sea level rise.

Two major episodes are evident in the metamorphic and igneous Precambrian basement of the Llano Uplift, central Texas. Dynamothermal metamorphism was accompanied by minor basaltic and tonalitic syntectonic plutonism. This was followed by a second period of thermal overprinting accompanying emplacement of high-K2O, high-level major granite plutons. Extensive isotopic age work by Zartman, published in the mid-1960s, suggests that development of the basement complex, spanning an interval of 150 m.y. or more, began with deposition of Valley Spring Gneiss (the lowest unit) and terminated about 1,050 m.y. ago with final postmetamorphic cooling (indicated by retention ages of Ar and Sr in biotite). We have supplemented these data with more than 50 new K-Ar and Rb-Sr analyses. Two foliated plutons in the southeast are 1,167±12m.y. (2σ) old, with distinctly different initial 87Sr/86Sr ratios. Field relationships and isotopic data indicate that these plutons are the earliest yet known in the Uplift. Metamorphosed basalt dikes and gabbro bodies were emplaced immediately preceding and following the syntectonic plutons. Eleven of these rocks had extremely uniform initial 87Sr/ 86Sr=0.7029±0.0005. A Rb-Sr whole-rock isochron of the unfoliated Enchanted Rock pluton indicates an age of 1,048±34 m.y. with initial 87Sr/86Sr= 0.7048±0.0007. One of the northern unfoliated granites, the Lone Grove pluton, gives a whole-rock isochron age of 1,056±12 m.y., with initial 87Sr/86Sr = 0.7061±0.0003. All of the intrusive rocks have initial 87Sr/86Sr ratios consistent with a source in the mantle or lower crust, but not in ancient remobilized continental crust. Six K-Ar hornblende ages from metabasalts are 1,078±19 m.y. (1σ), in general agreement with K-Ar and Rb-Sr mineral ages elsewhere in the eastern Llano Uplift. A metasedimentary Valley Spring Gneiss sample from the western Uplift has a whole rock-muscovite Rb-Sr age of 1,129±9 m.y. Field and isotopic data are now sufficiently numerous to permit a moderately detailed reconstruction of the Precambrian history of the area.

TJB Resources recently completed gravity, magnetotelluric, and seismic geophysical surveys in San Saba and Mills Counties north of the Llano uplift, west of the Ouachita Disturbed belt. Significant results show overthrusting has occurred 60-70 mi west of the Ouachita Disturbed belt. Earliest thrusting could have occurred in the Silurian-Devonian from south or southeast. Faulting reoccurs during Pennsylvanian Ouachita thrusting and again in the Late Cretaceous. The primitive Llano uplift buttressed the allochthonous rocks moving north and west. Magnetotelluric and gravity data indicate the Llano uplift is a separate and distinct isolated remnant or terrain, with numerous igneous intrusions reaching to great depths. Associated with this thrusting is a major tectonic northwest-southeast lineament located parallel to the termination of the Cretaceous rocks, outcropping on the western edge of Mills and Lampasas Counties. This lineament is documented by east-west seismic lines displaying a 0.3-sec displacement and farther southwest a 1.1-sec displacement. Paleozoic fossils were found near the surface trace of the northwest-southwest lineament as observed on the east-west seismic line. This area was previously mapped as Cretaceous. Surface inspection found Paleozoic rocks containing crinoids, spirifers, and rugose corals, with beds dipping to 45/sup 0/, contrasted with the N2/sup 0/ regional dip surroundingmore » the lineament area. Well-log correlations from Marble Falls to Ellenberger range from 350 to 600 ft from east to west near the Llano uplift. The 60 to 70-mi frontal edge of the thrusting provides potential hydrocarbon traps along a north to northeast trend from central Texas to Oklahoma.« less

The Marble Falls Formation (Lower Pennsylvanian) records normal marine conditions and widespread limestone deposition over the Llano uplift and the adjacent Concho platform in Texas. Examination of depositional environments of the outcropping Marble Falls facilitates interpretation of the oil-producing Lower Pennsylvanian shelf and shelf-edge carbonate rocks along the western margin of the Fort Worth basin. Most of the outcropping Marble Falls can be divided into two units separated by an unconformity. Lower Marble Falls is entirely Morrowan. Upper Marble Falls becomes younger westward: Morrowan in the east and Atokan in the west. Marble Falls deposition began with establishment of an open marine platform centered at the Llano uplift. Incipient calcarenite shoals developed at some slight break in slope. The northeast part of the platform resembled the modern Bahamian Platform, although platform-off platform relief was less than 9 m. Platform margins were approximately coincident with present outcrops on the north and east sides of the Llano uplift. The upper Marble Falls was deposited primarily as algal buildups and calcarenite shoals and as shale and spiculitic biomicrite in topographic lows. While the older lower Marble Falls Platform was subaerially exposed, deposition continued on the off-platform shelf adjacent to the rapidly filling Fort Worth basin. Progressive subsidence of the old platform allowed these facies to onlap the erosional surface. Strawn deltas simultaneously prograded across the upper Marble Falls shelf from the east. Marine energy levels and depositional relief were less than during deposition of the lower Marble Falls. End_of_Article - Last_Page 479------------

Temperature measurements were made in the surface portion of a test well drilled in Central Texas. The well portion of a test well drilled in Central Texas. The well was drilled to 304 ft by the cable tool method and surface casing was set and cemented. Temperature observations were made in the 25-hour interval between termination of drilling and installation of casing. Subsequently, temperature data were obtained, at progressively longer time intervals, to a total time of 280 hours after cementing of the surface casing when deepening of the hole by the rotary method commenced. Field observations indicated that a hole drilled by the cable tool method is heated (the maximum observed anomaly was about 5F); and hydra- tion of the cement leads to a general heating of the well. A maximum temperature of about 15F was, observed 14 hours after cementing was complete. Field measurements were supplemented by a laboratory study. Both the thickness of the cement annulus and the thermal diffusivity of the surrounding formation were varied. Results indicated that both parameters have a pronounced influence on the resulting temperature pronounced influence on the resulting temperature anomaly. Thus, an interpretation of the temperature log in terms of cement thickness behind the casing is permissible only when the lithology of the formation is reasonably uniform. Introduction The test well, Bill Stribling No. 3 (Fig. 1 ), is located in Central Texas over the Llano uplift where the sedimentary cover over the igneous basement is about 1,240 ft. The well was cable tooled to 304 ft between Sept. 22 and Oct. 27, 1964. The cable tool method was chosen since cavernous zones posed serious problems in a previous attempt with the rotary method (the bit dropped 6 ft previous attempt with the rotary method (the bit dropped 6 ft at one stage and circulation loss could not be controlled); 13-in. casing was set to 80 ft and 10-in. casing to 128 ft.. but neither casing was cemented. At noon on Oct. 27, drilling was suspended. Instrumentation All field measurements were taken with a 1,500-ft multiconductor rubber cable. Twelve glass bead thermistors are molded into the lower part of this cable at 25-ft intervals. Over the range of temperatures encountered in the shallow subsurface, these thermistors show a change of electrical resistance of about 100 ohm/ deg. F, Resistance for each thermistor is measured while the cable is at rest. Measurements are made with a four-digit bridge and an electronic direct-current null detector. Accuracy of the measurement is within 1/2 ohm. The cable resistance is measured separately for each cable position and subtracted from the reading. After the resistance of all thermistors has been determined, the cable is moved the desired distance-generally not less than 5 ft-and the next sequence of measurements is taken. Since the thermistors are insulated from the borehole fluid by about 0.1 in. of rubber, their time constant (95 percent recovery) is several minutes. Thus, a waiting period of 5 to 10 minutes is necessary before reliable measurements can be taken. As a result, each temperature log took about an hour to complete. This makes the tool ill-suited for the observation of transient temperature conditions as described in this article. A continuous logger such as the High Resolution Thermometer* or the one described by Simmons would be superior. Field Measurements Temperature measurements in the Stribling No. 3 well are summarized in Fig. 2. JPT P. 147