Martinsburg Formation Research Articles

Composite magnetic anisotropy fabrics: experiments, numerical models and implications for the quantification of rock fabrics

Magnetic fabrics from rocks with multiple mineral-preferred orientations may have anisotropy ellipsoids whose shape and orientation arise from the addition of two or more component fabrics. Our numerical models and experiments demonstrate that such composite magnetic fabrics do not directly reflect the shapes and/or orientations of the individual mineral fabrics and we provide criteria for the recognition and interpretation of composite fabrics in natural rocks. These criteria include: 1. (1) the orientation of the maximum susceptibility axis is located at the intersection of two planar fabrics, and 2. (2) the shape of the susceptibility ellipsoid changes from oblate to prolate and the degree of anisotropy decreases, as the relative intensity of two planar component fabrics becomes equal and as the angle between the planar fabrics increases. Composite magnetic fabrics are observed in the shales and slates of the Martinsburg Formation, Lehigh Gap, Pennsylvania. Modeling of the AMS (anisotropy of magnetic susceptibility) and ARMA (anhysteretic remanent magnetization anisotropy) behavior constrains the relative degree of anisotropy of the bedding-parallel and cleavage-parallel fabrics. In particular, ARMA model results allow a good estimate of magnetite fabric strength. We conclude that, in the presence of composite magnetic fabrics, quantitative measures of finite strain in deformed rocks are limited by the ability to accurately determine the degree of anisotropy and relative susceptibility of each component fabric. Such determinations require knowledge of the mineral(s) that are responsible for the measured magnetic fabric and their behavior during deformation.

Shear wave reflectivity, anisotropies, Poisson's ratios, and densities of a southern Appalachian Paleozoic sedimentary sequence

The physical properties of a sequence of Paleozoic sedimentary rocks have been examined in detail, with an emphasis on laboratory measurements of density, shear wave velocity, shear wave splitting, and V p / V s ratios. Seismic properties of 147 cores from 49 rock samples collected from the Thorn Hill sedimentary sequence of eastern Tennessee are examined in terms of implications for future seismic studies in the southern Appalachians. The shear wave velocities of these rocks are strongly influenced by the relatively high shear wave velocity of quartz. Shear wave velocity anisotropy is present in most of the lithologic groups; it is highest in the shales while being almost insignificant in the dolostones. The related phenomenon of shear wave splitting occurs to some degree in all of the lithologies studied and at high pressures originates from mineral orientation. Splitting, due to preferred mineral orientation, is most prominent in the shales, but also is significant in some of the carbonates. Compressional to shear velocity ( V p / V s ) ratios of approximately 1.82 (dolostones) and 1.95 (limestones) effectively characterize the carbonates while other lithologies display wider ranges of V p / V s , primarily due to the influence of accessory minerals such as quartz. Densities of the sample suite range from 2.34 g/cm 3 (shale) to 2.86 g/cm 3 (dolostone). Using average data for each lithologic group to calculate normal-incidence reflection coefficients, significant differences in the shear and compressional reflectivity of hypothetical interfaces are found. Normal-incidence shear and compressional wave synthetic seismograms approximating the subsurface reflectivity of the entire Thorn Hill section indicate that three zones of high amplitude reflections would be seen on reflection records obtained over this 3327-m-thick sequence. Overall, the shear and compressional synthetics are similar in appearance. Differences are seen at some interfaces in the Mississippian-Devonian interval, which are more reflective to shear waves, and in the Ordovician Martinsburg Formation, which appears more reflective to compressional waves.

Transmission Electron Microscope Observations of Illite Polytypism

AbstractTransmission electron microscopy (TEM), including selected area electron diffraction (SAED), has been used to identify polytypes in illite, phengite and muscovite from samples representing a wide range of diagenesis and low-temperature metamorphism. Samples include Gulf Coast sediments, sediments from the Salton Sea region, California, the Martinsburg Formation at Lehigh Gap, Pennsylvania, the Kalkberg Formation at Catskill, New York, Otago Schists from southern New Zealand, pelites from the Gaspé Peninsula in Quebec, Canada, shales and slates from Wales, sediments from the Barbados accretionary complex, and synthetic hydrothermal illite.Samples from rocks of lowest grades, including those representing a range of sedimentary diagenesis, invariably give SAED patterns with few, complex non-00l reflections which are diffuse and ill-defined and that represent largely disordered stacking sequences. Corresponding XRD patterns are consistent with 1Md polytypism. The term 1Md is therefore retained for this material. Higher grade samples, including those in which slaty cleavage is developed, and detrital grains in low-grade sediments invariably give diffraction patterns of well-ordered 2- or 3-layer polytypes. Of all samples and localities studied, only one diffraction pattern, from a sample in the Gaspé sequence, was found to be predominantly 1M. In none of the other sequences included in this study were any 1M or predominantly 1M electron diffraction patterns obtained for illite grains.Where illite is in its original state of formation, it is consistently 1Md, whether it originates as a result of direct crystallization from solution or as a replacement of smectite. Where illite has apparently undergone subsequent change, presumably through dissolution and crystallization representing an Ostwald-step-rule-like change, it occurs as a well-ordered 2-layer (inferred to be 2M1) or, less commonly, a 3T polytype. On the basis of this limited survey, the state of polytypism appears to directly identify illite as either being in, or changed from, its initial state of formation.

Open-system, constant-volume development of slaty cleavage, and strain-induced replacement reactions in the Martinsburg Formation, Lehigh Gap, Pennsylvania

No important changes in the volume of mudrocks occur across the mudstone-to-slate transition in the Martinsburg Formation at Lehigh Gap, Pennsylvania. Mass-balance calculations based on chemical analyses and specific-gravity measurements of 48 mudstones and 26 graywackes from the Martinsburg Formation across this 130-m strain gradient show constant ratios of Al2O3, TiO2, FeOT, MgO, MnO, Y, V, and Zr. This reflects the relatively low differential mobility of these components during diagenesis and slaty cleavage development, as well as the uniformity of compositions of the mudstones and graywackes at deposition. Using minimum-strain samples and Al2O3 as references, the calculations show no loss in SiO2, but large losses of CaO, Na2O, Ba, and Sr from the outcrop that are directly proportional to strain, and reflect an open system on the scale of kilometers. Analyses of graywacke-mudstone metagraywacke-slate) pairs show that losses in SiO2, Na2O, and volume, and gains in K2O and Ba in the graywacke/metagraywacke are balanced by opposite changes in the adjacent mudstone/slate. This documents a local mobility of SiO2 on the scale of centimeters. The inertness of most major components over the 100-m outcrop (especially SiO2), however, requires (1) that the composition of the fluid passing through the rocks was closely buffered by the quartz-albite-muscovite-chlorite assemblage, (2) that the volume of the fluid was small, and (3) that the P-T gradient down which it traveled was gentle. The changes in chemical composition identified can be explained by the inferred replacement reaction plag + 1M musc + K+ = 2M musc + qz + Ca+2 + Na+ + Sr+2 + Ba+2 The strong correlation of minor-element chemistry and strain indicates that this reaction was strain induced and that changes in bulk chemical composition are defined by the progress of this reaction.

Slaty cleavage development and magnetic anisotropy fabrics

The shale‐to‐slate transition preserved in the Ordovician Martinsburg Formation at the Lehigh Water Gap, Pennsylvania, provides an opportunity to study the relationship between magnetic anisotropy fabrics and the development of slaty cleavage. Our previous work has indicated that anisotropy of magnetic susceptibility (AMS) does not record changes in finite strain associated with cleavage development in these rocks but instead measures the degree of dissolution and new growth of chlorite. Additional AMS data presented in this paper lend further support to this conclusion. Conversely, anhysteretic remanent magnetization anisotropy (ARMA), which is not affected by paramagnetic chlorite, accurately reflects the strain‐induced rock fabrics associated with cleavage formation. ARMA results show that magnetite dimensional orientations vary from bedding‐parallel in shale samples to cleavage‐parallel in samples with well‐developed slaty cleavage. Samples with weak and pencil cleavage display scattered ARMA orientations which lie in between bedding and cleavage. These intermediate orientations may be due to either passive rotation of magnetite from bedding‐parallel to cleavage‐parallel or (re)crystallization of magnetite. If rotation occurred, grain rotation was highly heterogeneous in the samples with incipient cleavage. The intermediate ARMA orientations may also reflect the varying contribution of two magnetite preferred orientations, a depositional orientation parallel to bedding and a new growth orientation parallel to cleavage.

The Shochary Ridge sequence, southeastern Pennsylvania—a possible Ordovician piggyback basin fill

The central Appalachian orogen of eastern Pennsylvania is underlain, in part, by three fault-bounded deep water clastic sequences: the Windsor Township Formation of the Hamburg klippe (Greenwich slice), the Martinsburg Formation and the Shochary Ridge sequence. Deep water sandstones of the Windsor Township Formation were deposited in a trench on oceanic lithosphere in early Middle Ordovician time. Clastic detritus of the late Middle-early Late Ordovician Martinsburg Formation accumulated as part of a longitudinal deep sea clastic system in the central Appalachian foreland basin. Deposits of the coeval Shochary Ridge sequence comprise a 1500 m thick coarsening-upward sequence of hemipelagic mudstone and siltstone turbidites. Although devoid of the type of bedding cyclicity implicit to published submarine fan models, the Shochary Ridge sequence may have accumulated as part of a morphologically poorly developed deep sea fan in which submarine channels and depositional lobes never formed or were poorly developed at best. The coarsening-upward nature of the Shochary Ridge sequence probably reflects progradation of its depositional system in response to either an inferred late Middle-early Late Ordovician glacially induced reduction in sea-level and/or an increase in tectonic activity in the hinterland. Sedimentologic and paleocurrent data suggest that deposits of the Shochary Ridge sequence did not accumulate as part of the Martinsburg depositional system. Palinspastic restoration places the Shochary Ridge depocenter to the southeast of the Martinsburg basin; i.e., on the orogen side of the foreland basin. Accordingly, the Shochary Ridge sequence is interpreted to reflect gradual filling of a basin that existed within the active thrust terrane to the southeast of the Martinsburg depocenter. This basin, a “piggyback” or slope basin, formed on Martinsburg strata that had been tectonically incorporated into the northwest migrating thrust terrane as the frontal thrust advanced into the foreland basin.

Chlorite control of correlations between strain and anisotropy of magnetic susceptibility

Anisotropy of magnetic susceptibility (AMS) has in the past been used as an indicator of finite strain; mathematical relationships between the measured AMS ellipsoid and the finite strain ellipsoid in slates are numerous. The Ordovician Martinsburg Formation at Lehigh Gap, Pennsylvania, provides a unique opportunity to test this correlation, as a complete shale-to-slate transition is preserved. Magnitudes of the principal susceptibility axes appear to reflect a strain gradient at this location. Apparent strain values derived from AMS, using equations from Rathore and Hrouda, have X : Y : Z ratios from 2.79 : 1.87 : 0.19 (shale) to 2.92 : 0.82 : 0.42 (slate). The orientations of the susceptibility axes characterize two fabrics: (i) k int k max planes at 20° to bedding and (ii) k int k max planes parallel to cleavage. Anisotropy of magnetic susceptibility fails to show any intermediate orientations of the principal susceptibility axes in the zone in which cleavage is developed. Comparison with phyllosilicate crystallographic orientations indicates that the AMS intermediate and maximum susceptibility axes are parallel to chlorite basal planes. Because chlorite reoriented via dissolution and new growth as cleavage developed in this location, and because models correlating AMS and strain instead assume that strain is reflected by the rotation of the magnetic susceptibility carriers, the apparent strain values calculated from AMS reflect only the degree of dissolution and neocrystallization in these rocks and not the finite strain.

Sedimentology and evolution of the Martinsburg Formation (Upper Ordovician) fine‐grained turbidite depositional system, central Appalachians

ABSTRACTThe Upper Ordovician Martinsburg Formation of eastern Pennsylvania consists of mudstone, siltstone, and sandstone turbidites that accumulated in a tectonically active foreland basin. The mudstone‐rich Bushkill Member, the stratigraphically lowest unit of the Martinsburg in this area, grades upward into approximately equal proportions of mudstone, siltstone, and sandstone of the Ramseyburg Member. Many of the turbidites of these units are arranged in small‐scale (1–9 m) fining‐upward sequences that are interpreted as reflecting the influence of external or allocyclic controls such as variations in the local rate of sea‐level rise and/or variations in the intensity of tectonic activity in shelf/nearshore or hinterland areas rather than more commonly cited autocyclic mechanisms. The thick (approximately 2000 m) Bushkill‐Ramseyburg coarsening‐upward sequence records progradation of a muddy turbidite depositional system along the axis of the foreland basin. Although this sequence accumulated during a Caradocian eustatic rise in sea‐level, sedimentation rates landward of the shoreline were apparently great enough to allow for long‐term seaward progradation of the shelf source. The paucity of depositional lobe‐like facies (coarsening‐upward sequences) in the Bushkill Member allows for tentative comparison of the progradational Bushkill‐Ramseyburg system with the active fan lobe of the Mississippi Fan.Progradation of the Bushkill‐Ramseyburg system ceased abruptly when mudstone turbidites and laminated black shale of the upper unit of the Martinsburg, the Pen Argyl Member, accumulated. The great thickness of some mudstone turbidite beds of the Pen Argyl Member is interpreted to record topographic confinement of the central Appalachian foreland basin, which may have helped to preclude continued progradation of the Bushkill‐Ramseyburg turbidite system.

Sedimentology and possible paleoceanographic significance of mudstone turbidites and associated deposits of the Pen Argyl Member, Martinsburg Formation (Upper Ordovician), eastern Pennsylvania

The Pen Argyl Member, the stratigraphically highest unit of the Upper Ordovician Martinsburg Formation of eastern Pennsylvania, consists of the three mudstone facies: laminated mudstone, uniform mudstone, and black shale. These deposits conformably overlie a coarsening-upward sequence of fine-grained and classic sandstone turbidites of the underlying Bushkill and Ramseyburg Members, respectively. Petrographic and X-radiographic analysis suggests that laminated and uniform mudstone facies beds accumulated from muddy turbidity currents. Interbedded black shale facies beds, however, are interpreted to reflect slow, gravitational settling of sediment in a density stratified, anoxic basin. The marked transition from sandstone and siltstone turbidite deposits of the Ramseyburg Member to mudstone turbidite and black shale deposits of the Pen Argyl Member may reflect the combined effects of a Caradocian rise in sea level (increased organic-carbon production; lack of sand-size sediment) and a tropical to subtropical location of the basin (density stratification). Such a model, however, does not explain adequately the very thick laminated and uniform mudstone facies beds common only to the Pen Argyl Member. An alternative hypothesis involves amplification of global eustatic- and climatic-related oceanographic effects by local or basinal influences. The very thick beds of the laminated and uniform mudstone facies, by comparison with lithologically similar deposits of the eastern Mediterranean, are interpreted to have been ponded in a topographically restricted basin. Accordingly, deposition of the Pen Argyl Member may record a significant change in basin morphology from a wide extensive basin to a topographically restricted or enclosed anoxic basin.

The Valley and Ridge Province of eastern Pennsylvania—stratigraphic and sedimentologic contributions and problems

AbstractMany contributions that have led to a better understanding of Appalachian geology have resulted directly from work in the folded Appalachian Mountain and Great Valley sections of the Valley and Ridge physiographic province of eastern Pennsylvania. Disagreements have been common since H.D. Rogers first described the geology of the area in 1858. Many differing opinions still exist regarding the stratigraphy, structural geology, geomorphology, and glacial geology.The rocks in the area, which range from Middle Ordovician to Late Devonian in age, are more than 25000 feet (7620 m) thick. This diversified group of sedimentary rocks was deposited in many different environments, ranging from deep sea, through neritic and tidal, to alluvial. In general, the Middle Ordovician through Lower Devonian strata are a sedimentary cycle related to the waxing and waning of Taconic tectonism. The sequence began with a greywacke‐argillite suite (Martinsburg Formation) representing synorogenic basin deepening. This was followed by basin filling and pro‐gradation of a sandstone‐shale clastic wedge (Shawangunk Formation and Bloomsburg Red Beds) derived from the erosion of the mountains that were uplifted during the Taconic orogeny. The sequence ended with deposition of many thin units of carbonate, sandstone, and shale on a shelf marginal to a land area of low relief. Another tectonic‐sedimentary cycle, related to the Acadian orogeny, began with deposition of Middle Devonian rocks. Deep‐water shales (Marcellus Shale) preceded shoaling (Mahantango Formation) and turbidite sedimentation (Trimmers Rock Formation) followed by another molasse (Catskill Formation).

Gradient versus cluster analysis of fossil assemblages: a comparison from the Ordovician of southwestern Virginia

Studies in modern ecology indicate that most species are distributed independently along environmental gradients according to their individual requirements. Steep gradients often produce species associations separated by discontinuities; gradual gradients produce broadly-overlapping distributions. Approaching the distribution of species populations as a continuum, using gradient analysis, avoids artificial subdivision of totally intergrading distributions, yet permits discontinuities to emerge where present. Faunas of the Martinsburg Formation (Ordovician) in southwestern Virginia offer an excellent opportunity to test the applicability of gradient analysis in a paleoecological setting. A broad spectrum of environments, from nearshore to open-marine, clastic to carbonate-dominated facies, provide both temporal and geographic variation against which to evaluate changes in species distributions. Variations of five classical, Petersen-type communities were recognized in the Martinsburg using cluster analysis: (1) Lingula, (2) bivalve, (3) Rafinesquina, (4) Onniella, and (5) Sowerbyella-dominated communities. Two gradient analysis techniques, ordination and Markov analysis, revealed the same basic associations. However, ordination and Markov analysis permit arrangement of these associations along one or more interpreted environmental gradients. Factors related to water depth and distance from clastic source areas, particularly bottom stability and disturbance frequency, appear to have been the most important of a complex of interrelated physical parameters. The high-stress, nearshore end of the Martinsburg gradient complex was occupied by a Lingula association, followed seaward by an association of bivalves adapted to less-stressed environments. Low-stress, open-shelf environments were occupied by Rafines-quina, Onniella, or Sowerbyella-dominated associations. Broad overlaps among these articulate brachio-pod associations reflect variations in the open-shelf habitat.

Determination of fold kinematics from syntectonic fibers in pressure shadows, Martinsburg Slate, New Jersey

Determination of fold kinematics from syntectonic fibers in pressure shadows, Martinsburg Slate, New Jersey

Problems of volume loss, fabric development and strain determination in low-grade pelitic rocks: Martinsburg Formation, U.S.A. : D. R. Gray, Department of Earth Sciences, Monash University, Clayton, Victoria, Australia, 3168, and T.O. Wright, Earth Sciences Division, National Science Foundation, Washington, D.C. 20550, U.S.A.

Problems of volume loss, fabric development and strain determination in low-grade pelitic rocks: Martinsburg Formation, U.S.A. : D. R. Gray, Department of Earth Sciences, Monash University, Clayton, Victoria, Australia, 3168, and T.O. Wright, Earth Sciences Division, National Science Foundation, Washington, D.C. 20550, U.S.A.

Textures in Layered Silicates: Progressive Changes Through Diagenesis and Low-Temperature Metamorphism

ABSTRACT Phyllosilicates in samples from Gulf Coast cores and the Martinsburg Formation near Lehigh Gap, Pennsylvania (which together represent a continuous sequence from mudstone to slate) are shown by TEM studies to possess imperfections on a massive scale and to exhibit a progressive increase in perfection through diagenesis and low-grade metamorphism. In shallow portions (1,750 and 2,450 m) of the Gulf Coast core, smectites have irregular and curved layers with high dislocation densities and many small-angle, grainlike boundaries. In both the deeper portion (5,500 m) of the Gulf Coast sediments and in the Martinsburg mudstone samples, gram perfection increases in illites, but a complex mosaic structure with small-angle grain boundaries and edge dislocations still prevails. In the Martinsbu g slate, phyllosilicates exhibit layers which are continuous and straight and have few imperfections. Original clays which form at low temperatures have highly imperfect and metastable structures. Changes with increasing diagenesis are akin to annealing processes, with increasing perfection of grains being an approach to a more stable, strain-free structure. Temperature, pore water, time, and other variables affecting these changes act only to promote reaction rates to the defect-free, stable structures.

Petrographic Trend-Surface Analysis of Martinsburg Sandstones of Eastern Pennsylvania and New Jersey: Significance and Implications: ABSTRACT

The Martinsburg Formation (Middle-Upper Ordovician) of eastern Pennsylvania and New Jersey, part of a flysch belt that extends from Newfoundland to Alabama, records Taconic orogenic events and foreland basin sedimentation in the central Appalachians. Facies analysis of the Martinsburg in eastern Pennsylvania and western New Jersey suggests that it was derived predominantly from longitudinal sources and did not accumulate as a typical submarine fan, as commonly believed. Petrographic trend-surface analysis of 45 samples of Martinsburg sandstone from Delaware Gap, New Jersey, to southeastern New York indicates: (1) an increase in total quartz content to the northeast; (2) a decrease in lithic fragment content to the northeast; and (3) a decrease in total feldspar content to the northeast. These results are consistent with overall compositional trends within the Martinsburg belt from southwest Virginia to New York, described in previous studies, and support a dominant source terrane to the southwest. Significantly, trend-surface analysis points out a lack of strike-parallel concentrations of specific framework components, thereby reinforcing a longitudinal dispersal system. Consideration of the Martinsburg sandstone in terms of published QFL diagrams suggests derivation from a recycled orogene source characterized by uplifted supra-crustal sedimentary and lesser plutonic/low-grade metamorphic rocks, and points out the lack of a major volcanic input. This is further substantiated by preliminary geochemical examination of these sandstones, which supports sands one accumulation at a tectonically active but non-volcanic margin, although volcanic arcs may have existed outboard (southeast) of the Martinsburg source terrane. End_of_Article - Last_Page 1449------------

Dynamic analysis of rough cleavage in the Martinsburg Formation, Maryland : Onasch, C M J Struct GeolV5, N1, 1983, P73–81

Dynamic analysis of rough cleavage in the Martinsburg Formation, Maryland : Onasch, C M J Struct GeolV5, N1, 1983, P73–81

Shelf Sedimentation Above Storm Wave Base in Upper Ordovician Reedsville Formation in Central Pennsylvania: ABSTRACT

Interbedded fine-grained sandstone and shale in a generally coarsening-upward sequence characterize the Reedsville Formation. Data on sedimentary structures, lithology, bedding characteristics, and fossils for eight measured stratigraphic sections indicate that most beds were deposited by occasional storm-generated currents. Storm facies exhibit (1) abundant winnowed shell lags coupled with low-angle cross-stratified finer sediment, (2) abrupt lateral thickness variation in many beds, (3) sharp, erosive upper and lower bed contacts, and (4) well-preserved, unabraded outer sublittoral benthic fauna. Hummocky cross-stratified beds are common, and in many places are associated with wave-rippled sandstones. A shallow open-shelf environment is inferred. Storms of variable intensity and duration periodically scoured and suspended bottom sediments and deposited individual fining-upward units under conditions of strong but waning bed shear. An overall increase in the sandstone/shale ratio from bottom to top in the progradational sequence suggests gradual shallowing and more frequent storm-wave influence. Uppermost beds contain intertidal fossil communities. Paleocurrent data indicate east- and northeast-directed sediment transport. These interpretations are not consistent with the common assumption that the Reedsville is simply the deep-basin, distal equivalent of the adjacent Martinsburg Formation. The present data suggest that Reedsville sediments accumulated on a shelf west of a structural hinge line that comprised the western margin of the thicker, deeper water Martinsburg sequence. Differential subsidence across this shelf-edge hinge line may account for the significant differences in paleobathymetry of the two formations. End_of_Article - Last_Page 1917------------

Interaction of bed-parallel stylolites and extension veins in boudinage

A form of discontinuous boudinage (styloboudinage) involves extension along and shortening across layers due to the contemporaneous development of extension veins and localized bed-parallel stylolites. Styloboudins develop under anchimetamorphic conditions and occur in the thinnest parts of hummocky-bedded limestone layers enclosed by calcareous mudrock within the Martinsburg Formation of southwest Virginia, U.S.A. They show complex subfabrics defined by sets of orthogonal veins and stylolites. The boudins are late-stage deformation structures associated with reactivation of early bed-parallel stylolites and development of bed-normal calcite veins. Pressure-solution shortening across these stylolites is 10–15% whereas finite elongations recorded by veins ranges from 40–70% in thin parts to 10–25% in the widest parts of layers. Latest pull-apart and veining in some cases utilized existing stylolites at high angles to bedding.

Origin and significance of microstructures in sandstones of the Martinsburg Formation, Maryland

Origin and significance of microstructures in sandstones of the Martinsburg Formation, Maryland

Sedimentation rates of the Taconic sequence and the Martinsburg Formation

The fast-activating/deactivating voltage-gated potassium channel Kv3.3 (<i>Kcnc3</i>) is expressed in various neuronal cell types involved in motor function, including cerebellar Purkinje cells. Spinocerebellar ataxia type 13 (<i>SCA13</i>) patients carrying dominant-negative mutations in <i>Kcnc3</i> and <i>Kcnc3</i>-null mutant mice both display motor incoordination, suggested in mice by increased lateral deviation while ambulating and slips on a narrow beam. Motor skill learning, however, is spared. Mice lacking <i>Kcnc3</i> also exhibit muscle twitches. In addition to broadened spikes, recordings of <i>Kcnc3</i>-null Purkinje cells revealed fewer spikelets in complex spikes and a lower intraburst frequency. Targeted reexpression of Kv3.3 channels exclusively in Purkinje cells in <i>Kcnc3</i>-null mice as well as in mice also heterozygous for Kv3.1 sufficed to restore simple spike brevity along with normal complex spikes and to rescue specifically coordination. Therefore, spike parameters requiring Kv3.3 function in Purkinje cells are involved in the ataxic null phenotype and motor coordination, but not motor learning.