Magnetostratigraphy and tephrochronology of an upper Pliocene to Holocene record in lake sediments at Tulelake, northern California

HUGH J. RIECK* U.S. Geological Survey, Flagstaff, Arizona 86001 ANDREI M. SARNA-WOJCICKI \ CHARLES E. MEYER 1 U.S. Geological Survey, Menlo Park, California 94025 DAVID P. ADAM j

ABSTRACT correlation. Most of the tephra at Tulelake records eruptions from the nearby southern and central Cascade Range of Oregon and northern Combined paleomagnetic and tephra chronologies of one of the California, and the Medicine Lake Highland of northern California. most complete middle Pliocene through Holocene stratigraphic rec- Deposition took place during most of the past 3 m.y. within the Tule- ords yet recovered in western North America provide a reference lake basin; notable periods of slow or sporadic accumulation, or ero- section for much of northwestern North America and adjacent Pacific sion, occurred between about 620,000 and 200,000 yr B.P. and Ocean. Five long drill cores of lacustrine sediments at Tulelake, north- between about 2.5 and 2.1 Ma. Rapid deposition occurred during ern California, recovered a nearly continuous 331-m-thick record marine oxygen-isotope stage 6, between about 170 and 125 ka. Re- spanning the past 3 m.y. The Brunhes Normal-Polarity, Matuyama gional volcanism during the past 3 m.y. was markedly episodic, with Reversed-Polarity, and Gauss Normal-Polarity Chronozones are rec- notable volcanic activity from about 2.1 to 1.9 Ma and from 0.4 Ma to ognized; within these, the Jaramillo, Olduvai, Reunion(?), and the present. Kaena(?) Subchronozones are present. Six short stratigraphic inter- vals exhibit anomalous remanent inclinations that may record excur- INTRODUCTION sions and brief subchrons within the Brunhes and Matuyama Chronozones. Age estimates suggest correlation of five of the anoma- Detailed geologic records of late Neogene paleoclimates and past lous intervals with (1) one of the Biwa excursions at about 18,000 yr volcanic activity are becoming increasingly important to earth scientists B.P., (2) the Mono Lake excursion at about 27,000 yr B.P., (3) the attempting to determine the history and natural variability of climate Blake Reversed-Polarity Subchron at about 114,000 yr B.P., (4) the systems. Late Neogene climatic reconstructions and global-circulation Kamikatsura Normal-Polarity Subchron at about 850,000 yr B.P., and models are being developed to predict the timing and extent of climate (5) the Cobb Mountain Normal-Polarity Subchron at about 1.10 Ma. changes that may be anticipated in the near future. Concurrently, the Age of the sixth interval of anomalous inclination is broadly con- possible relation of volcanic activity and climatic response is being investi- strained between 117,000 and 180,000 yr B.P. gated. The predictive models must rely heavily on inferences drawn from Sixty-three individual tephra layers were characterized by geologic records. Viable attempts to reconstruct ancient ocean and electron-microprobe and X-ray fluorescence analyses of volcanic glass atmosphere-circulation patterns during climatic episodes significantly shards. Identified tephra of relatively well known age include (1) the warmer than any during the late Pleistocene must be based on an extensive basal airfall pumice at Llao Rock, 7015 yr B.P.; (2) the Trego Hot paleoclimatic data base from both marine and continental records. De- Springs Bed, 23,400 yr B.P.; (3) the Olema ash bed, between 55,000 tailed age control and precise correlation are essential for meaningful and 75,000 yr B.P.; (4) the airfall pumice at Goudcap Road ("Pumice interpretation of such records. Castle-like tephra 2"), about 120,000 yr B.P.; (5) the Rockland ash Five cores were drilled by the U.S. Geological Survey in a thick bed, about 410,000 yr B.P.; (6) the Lava Creek-B ash bed, 620,000 yr sequence of lacustrine sediments at Tulelake, northern California (Fig. 1). B.P.; (7) the Rio Dell ash bed, about 1.45 Ma; and (8) the Bear Gulch The objectives of drilling were to (1) provide a terrestrial record of late ash bed, about 1.9 Ma. A sedimentation-rate curve based on inde- Pliocene, Pleistocene, and Holocene climate change, (2) obtain a record of pendently dated tephra and polarity reversals is used to infer age late Cenozoic volcanic activity, and (3) establish a regional chronostrati- estimates of undated or previously unidentified ash beds. Some of graphic reference framework through which other, shorter or less complete these ash beds are found over large areas of the western United States terrestrial and marine records of paleoclimatic and volcanological events and eastern Pacific Ocean basin and provide widespread horizons for could be placed into temporal context. This temporal framework, together with geochemical, paleontological, and other studies (for example, Adam and others, 1986a, 1986b, 1987,1989), provides a basis for evaluation of * Present address: U.S. Geological Survey, M.S. 913, Box 25046, Federal large ancient lake systems, remnants of which are preserved over much of Center, Denver, Colorado 80025-0046. the western United States. The 331-m-thick record recovered at Tulelake

Additional material for this article (Table A) may be secured free of charge by requesting Supplementary Data 9209 from the GSA Documents Secretary.

Geological Society of America Bulletin, v. 104, p. 409-428, 8 figs., 4 tables, April 1992.

409

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121 30 W

OREGON Figure 1. Location of Tulelake LU drill site. Dashed line indicates ap- CD proximate limit of untilted lake sed- < iments. Solid lines and hachured < O CALIFORNIA < line mark traces of prominent > UJ north-northwest normal faults. QC Note that the Clear Lake shown here is not the same as the Clear LÜ Lake mentioned in the text, which is in Lake County, about 350 km to O < the south-southwest of Tulelake. OREGON O 42° N CO TULELAKE CALIFORNIA < • DRILL , , SITE i \ o CLEAR LAKE

LAVA BEDS NAT'L. MON. (Quoternary basalts)

0 10 20 km offsets (McKee and others, 1983). Development of fault-scarp ridges, ^^ | MEDICINE coupled with damming of southward drainage by volcanic deposits, is LAKE thought to be responsible for the accumulation of at least 550 m of lake HIGHLANDS sediment in the Tulelake basin (Adam and others, 1986a, 1986b, 1987). The sedimentary record of this basin probably extends back to about the is one of the longest and most complete late Neogene stratigraphie records Miocene-Pliocene boundary. Deposition continued in the basin into his- from the western United States. toric time, when the lake was drained for fanning. We present here integrated paleomagnetic and tephrostratigraphic Five cores were drilled within about 30 m of each other in the town chronologies for the 3-m.y. record of events preserved in the Tulelake of Tulelake, at an elevation of about 1,220 m (Fig. 1). The drill site was section. The combined chronologies aid in deciphering late Neogene basin located close to the center of the Tulelake basin, near the Tulelake munici- development in the region, facilitate correlation of paleoclimatic informa- pal water well, for which the driller's log showed an ~500-m-thick se- tion among numerous terrestrial basins in the western United States, and quence of lake sediments. Overall, about 90% of the depositional sequence link contemporaneous deposits of the western coterminous United States was recovered. Short stratigraphic intervals from which core could not be and eastern Pacific Ocean. Results allow us to provide age estimates and recovered in one hole were often recovered in another; recovery was identify probable eruptive source areas for several tens of previously un- virtually complete for the multiple, overlapping cores in the upper 80 m identified tephra layers that may be widespread chronostratigraphic (Fig. 2). Bedding at the surface is horizontal, and consistent paleomagnetic markers and also provide a history of volcanism in the northwestern inclinations at depth, similar to those expected from horizontal beds, sug- United States for this time interval. gest that no appreciable tilting has taken place in the lower part of the cored section. GEOLOGIC SETTING Most of the cored section consists of mud and diatomite, although marl and a few slightly sandy and tuffaceous intervals also were noted Tulelake lies near the western margin of the Modoc Plateau (Mac- (Fig. 3). Regional volcanism, particularly in the southern and central donald, 1966), a region physiographically transitional between the Cascade Range of northern California and Oregon and in the Medicine southern Cascade Range to the west and the Basin and Range province to Lake Highland of northern California, has resulted in numerous tephra the east. A thick and little known sequence of interbedded upper Miocene layers throughout much of the section. A number of these are widespread to lower Pliocene basalt flows and sedimentary deposits underlies much of silicic tephra layers identified and independently dated at other sites, and the region. The overlying Pliocene through Holocene volcanic and sedi- they thus provide age control that serves to calibrate the magnetostrati- mentary rocks are widely exposed (McKee and others, 1983). Crustal graphic record at Tulelake. Conversely, the magnetostratigraphy and thinning by extension since middle Miocene time has produced numerous correlated dated tephra layers provide age constraints for previously un- north-northwest-trending normal faults with relatively small vertical dated tephra layers within the sediments of Tulelake.

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Depth Dominant Lilhology

Cm) O- Olive grciy clay — and silly clay • -WS-S • -¿¿k" •'••^gfji -lì 30- R « - ; ^ . >«» •:»» S . ,!r ••« •• — p'i'lVI; Olive grey and brown 60- riarly and silly clay

CORES DEPTH (m) i 2 3 4 5 90- Cray and o!ive gray tephra o—l and reworked tephra S3 120-

j.'ssr-y Ñ-xf-s- r ifftss" î< ÚÉX-- — " Olive gray, brown, and black 150- clay with organic material

180-

2 10-

Tephrg Occurrence S Thickness 240- — Less lhan 5 cm

270- -—— 5 to 20 cm

——— Wore than 20 cm 300- Distribution of

Core Recovery 330-

Figure 3. Generalized composite lithologie log of Tulelabe cores 1-5.

bjd m = Recovery METHODS

" I I= No Recovery A mobile rotary drilling rig with a wireline core barrel was used to obtain core. Preliminary samples were taken at the drill site for paleomag- netic, geochemical, palynologie, radiocarbon, and tephra analyses as time Gap= Recovery Not Attempted and coring progress permitted. Cores were sealed in plastic wrap and placed in PVC pipe for transport and storage. After preliminary analyses, Figure 2. Columnar diagram showing depths and core recovery the cores were resampled in the laboratory to develop more complete data of individual Tulelake cores 1 through 5. sets and confirm preliminary results.

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Paleomagnetic Sampling and Measurement trace elements allowed discrimination between similar tephra layers erupted from the same source areas, but internal variability of samples Paleomagnetic samples were collected in the manner described by could not be tested using bulk separates. Elemental concentrations in the Gravenor and others (1984). A sharp thin blade was used to carve cubes of selected separates were determined by least-square fits of their spectra to sediment from a flat surface shaved well into the side of the core. Standard those of previously determined U.S. Geological Survey standards. The acrylic plastic paleomagnetic sample boxes were then slipped over the elements used most often in comparing samples by XRF analysis were Ti, cubes of sediment, and the samples were removed from the core. Individ- Mn, Rb, Sr, Y, Zr, and Nb. ual sample boxes were sealed and refrigerated to minimize desiccation and Calculated oxide concentrations for the Tulelake samples generally oxidation. Remanent magnetizations were measured with a superconduct- totaled between 93% and 96%. Silicic glasses produced by modern erup- ing magnetometer that has a noise level less than 10"4 Am-1. Each sample tions generally have oxide totals of about 98% to 99% (Sarna-Wojcicki and was subjected to progressive alternating field (AF) demagnetization in at others, 1981). The lower totals of the Tulelake samples are due to the least 4 and as many as 12 steps. Peak demagnetizing field strengths ranged presence of fluids not measured in EMA, mostly water resulting from from 10 to 70 milliTesla (mT). Measurements of bulk magnetic suscepti- hydration of the glasses. Oxides were recalculated to 100% on a fluid-free bility and anisotropy of magnetic susceptibility were made using a low- basis to allow direct comparisons for tephra identifications. 5 field inductance coil that has a sensitivity of 10" (SI, dimensionless). Identifications and correlations of tephra layers were based on chem- Neither absolute nor relative azimuthal orientations can be confi- ical composition of their glass shards, supplemented in some instances by dently determined for the paleomagnetic samples because (1) coring was petrographic characteristics. Chemical correlations were evaluated using a done by the rotary wire-line method, making it difficult to determine similarity (SIMINAL) coefficient (Borchardt and others, 1972; Sarna- declination of core segments by means other than the paleomagnetic direc- Wojcicki, 1976; Sarna-Wojcicki and others, 1979, 1984), and standard tions themselves; and (2) evidence of differential rotation at bedding planes deviations of averages of ratios (RATIONAL) of element concentrations within individual core segments, probably undetected in many instances, (Sarna-Wojcicki and others, 1984). Elements that could be determined precluded confident reconstruction of relative declinations from the pa- most precisely, and that showed the greatest contrasts between different leomagnetic data alone. Inclination of the average geocentric axial dipole tephra layers, were used to identify correlative tephra layers. Average field at the drill site (±61°), however, allows paleomagnetic polarity to be precision for the elements used in matching samples was between 3% and determined from inclination data alone. 4%, resulting in similarity coefficients (S.C.'s) of about 0.96 to 0.98 for multiple samples of the same tephra layer from the same locality, or Tephra Sampling, Analysis, and Data Evaluation replicate analyses of one sample. The S.C. for a perfect match, for which analytical errors are zero, is 1.00. Similarity coefficients less than about Two types of samples were collected for chemical analysis of tephra 0.94 are not considered a strong support for correlation. Additional con- layers: (1) channel samples taken vertically across visible discrete tephra straints on the identities of tephra layers were obtained from stratigraphic layers and (2) spot samples taken where tephra was suspected on the basis relations, biostratigraphic age estimates in DSDP cores, and sedimen- of sedimentologic and other physical characteristics, or was suspected on tation-rate curves. the basis of stratigraphic position relative to identified horizons. Where visual inspection indicated multiple emplacement units within a single PALEOMAGNETIC RESULTS layer, each unit was sampled individually to test for multiple composi- tional modes. For detailed sample preparation procedure, see Sarna- Remanent Magnetizations Wojcicki (1976) and Sarna-Wojcicki and others (1979, 1984). Natural remanent magnetizations (NRM's) of paleomagnetic samples Glass separates were analyzed by electron microprobe analysis ranged in intensity from about 3 x 10~4 Am-1 to about 2 x 10 2 Am-1. (EMA) and, in some instances, energy-dispersive X-ray fluorescence Samples of normal polarity generally had NRM inclinations near that of (XRF) analysis. Shards were analyzed individually in EMA for oxides of the present field (Fig. 4A). Samples of reversed polarity generally had Si, Al, Fe, Mg, Mn, Ca, Ti, Na, and K. Conventional wet-chemical anal- shallow NRM inclinations and weaker dipole moments. After partial yses of natural glasses and silicate minerals were used as standards. alternating-field (AF) demagnetization, samples of normal polarity Corrections were made for background, atomic number, absorption, and showed little change in direction. Samples of reversed polarity commonly fluorescence using a modified on-line data reduction-program (Yakowitz steepened in negative inclination and some showed slight increases of and others, 1973). Generally, concentrations of Si, Al, Fe, Ca, Na, and K intensity in the lower demagnetizing fields (Fig. 4B). This behavior of the could be determined by EMA with a precision of 7% or better. These reversed polarity samples suggests removal of low-coercivity secondary elements were used most commonly in our comparisons. Concentrations components of normal polarity. of Mg, Mn, and Ti are often too low in silicic glasses for precise determina- Samples from intervals of reversed polarity show considerably more tion by EMA and were not generally used in correlation of silicic layers. In scatter in inclination than samples from intervals of normal polarity. Even more mafic samples, however, concentrations of Mg and Ti are often large after partial demagnetization, primary detrital remanent magnetizations enough to be reliably measured and compared. Because of the high mobil- (DRM's) were not isolated in many samples from the reversed polarity ity of Na and K, their concentrations can be variable within the same zones. Possible explanations include (1) high-coercivity components of tephra layer (Cerling and Brown, 1982). Thus, for comparison of some of viscous remanent magnetization (VRM) that could not be completely the older silicic tephra layers (in this report, about 1.5 Ma or older), we removed by AF demagnetization, (2) secondary components of high coer- omitted Na and K, and substituted Ti, if the analytical precision of the civity carried by diagenetic minerals, and (3) postdepositional detrital latter was sufficiently high. Minor oxides found from previous experience remanence (pDRM) acquired after reversal of the geomagnetic field but to be of little value in characterizing tephra samples, P2O5 for example, before dewatering and induration had completely fixed the orientation were not determined. of all magnetic grains in the sediment. Samples that consistently steep- Selected bulk glass separates of 0.5 g also were analyzed by XRF for ened in inclination along great-circle paths during demagnetization, but K, Ca, Ti, Mn, Fe, Cu, Zn, Rb, Sr, Y, Zr, and Nb. X-ray counts were made which had stable end-point directions undefined, are considered to have on ~ 15 shards per sample to evaluate homogeneity and to test for multiple been deposited during the polarity indicated by the direction of steepening compositional modes. The greater sensitivity of XRF for these minor and (Fig. 4C).

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W UP Figure 4. Representative vector demagnetization diagrams show- ing sample behavior during alternating-field (AF) demagnetization (NRM-80 mT). Declinations (dots) are uncontrolled (azimuthally un- oriented). X's mark inclinations. Axis divisions are 1 x 10-4 Am-1. (A) Normal polarity, single component of magnetization; (B) reversed polarity, secondary normal polarity component removed; (C) re- versed polarity, stable end point undefined; (D) unstable magnetiza- tion, results discarded.

120 350 tremely low Koenigsberger ratios (NRM/susceptibility <0.05). The low NRM 283.04 m 162.65 m Koenigsberger ratios may reflect a weak DRM signal because of reduced E DOWN E DOWN field intensities during excursions or polarity transitions. These three sam- ples, however, are stratigraphically isolated and, without further supporting N UP N UP evidence, cannot be assumed to have recorded field behavior accurately. After discarding obviously suspect results from the data set, some remaining samples displayed anomalous inclinations of apparently stable magnetization. Further investigation of these samples was prompted by (1) recognition of discrete horizons of anomalous inclinations from two separate samplings of the individual cores and (2) stratigraphic reproduci- bility of three of these horizons that were intersected by different cores. Single samples with anomalous inclinations of stable magnetization are shown in the stratigraphic plot but are disregarded in the interpretation. NRM —• Magnetic Fabric E

339 166 Physical deformation not visible in the cored sediments could cause 136.88 m .69 m anomalous remanent inclinations. Measurements of low-field anisotropy S DOWN S DOWN of magnetic susceptibility (AMS) were made to identify any alteration of magnetic fabric that would reflect otherwise undetected deformation. The results give no indication of deformation sufficient to account for the Reliability and Data Evaluation anomalous inclinations. Equal-area plots of the principal axes of suscepti- bility ellipsoids and of remanence directions during progressive AF de- Samples were assessed for reliability before we developed a polarity magnetization from samples across one zone of anomalous inclinations zonation. The most important criterion for reliability was consistency of (Mono Lake excursion) are shown in Figure 5. The highest and lowest behavior during demagnetization. Most samples remained stable through samples show expected results from immediately above and below the peak demagnetizing fields of 50 to 70 mT. Of the 447 paleomagnetic anomalous samples. Five of the six samples from within the interval dis- samples collected and analyzed, results from 23 samples were considered unreliable and were discarded. Sixteen of those, mostly from marly units, displayed erratic behavior during demagnetization (Fig. 4D); four samples had intensities too low for accurate determination of direction after partial Figure 5. Equal-area projections of demagnetization; and three samples with anomalous inclinations had ex- principle axes of magnetic susceptibility el- lipsoids (maximum, intermediate, and min- imum) and directions of remanence during progressive AF demagnetization (unla- beled) of two samples from two cores that intersected a stratigraphic interval of O INT anomalous remanent inclinations [Mono Lake(?) excursion]. A is from an interval of typical normal polarity remanent incli- nation. B shows anomalous remanent in- clinations. Declinations are uncontrolled + MIN W (azimuthally unoriented). Solid dots are lower hemisphere; open circles are upper hemisphere; X marks normal polarity geo- centric axial dipole ("expected") field di- rection. All samples from this interval 230 show near-expected susceptibility direc- 13.0 9.32 m tions, indicating undeformed magnetic 7.45 m fabric.

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Figure 6. Composite stratigraphic plot of remanent inclinations, intensities, and bulk magnetic susceptibilities after partial AF de- magnetization (10 to 50 mT), and interpreta- tion of polarity for the Tulelake cores. Horizons of polarity zone boundaries labeled in meters. Unreliable results from 23 samples not included; all others shown but results from single samples not used in interpreta- tion. Ages of key tephra identified here are independently determined elsewhere; these and other dated tephra support correlations with the polarity time scale.

200

6

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play anomalous remanent inclinations. All samples have weak, foliate about 3.01 Ma (Mankinen and Dalrymple, 1979). Together, these data anisotropy. Axes of maximum and intermediate suscepbitility are near suggest that the base of the cored section is close to 3.01 Ma. horizontal, and axes of minimum susceptibility are near vertical. These Correlation of polarity zones with a polarity time scale merely con- results are characteristic of undeformed, lacustrine detrital magnetic fabric strains the possible age range of strata within the identified zones (that is, (Ising, 1942; Marino and Ellwood, 1978; Hrouda, 1982). Remanence zone boundaries are not necessarily of the same age as the polarity directions do not parallel maximum susceptibility directions, indicating changes). Age control in the Tulelake cores, however, suggests that deposi- that orientation of the smaller, more stable (that is, "remanence-carrying") tion was sufficiently uniform that the zone boundaries may be regarded as grains was controlled by the field direction, whereas orientation of the virtually contemporaneous with the corresponding polarity changes. larger magnetic grains, which contribute little to remanence but much to Our correlation of the paleomagnetic record across the lower part of susceptibility, was dominated by sedimentary processes. the Matuyama Reversed-Polarity Chronozone (Figs. 6, 7a) suggests that major fluctuations in sedimentation rate occurred during deposition of that Bulk Magnetic Susceptibility interval. Sedimentologic, palynologic, and paleontologic evidence also in- dicate interruptions in accumulation across this interval (Adam and others, Measurements of bulk magnetic susceptibility of all samples deter- 1989). mined (1) whether the observed variations in remanent intensities resulted from fluctuations in intensity of the geomagnetic field or from variations in Magnetic Excursions magnetic mineralogy and (2) if the intervals of anomalous remanent incli- nations correspond with changes in magnetic mineralogy. In general, large Four short intervals within the Brunhes Normal-Polarity Chronozone changes in remanent intensity and susceptibility coincide in the Tulelake and two within the upper part of the Matuyama Reversed-Polarity cores (Fig. 6), indicating that both are controlled primarily by changes in Chronozone yielded multiple samples that have stable magnetizations, but magnetic mineralogy. On a detailed level, however, uniform susceptibili- directions that suggest polarities opposite to those expected (Fig. 6). Be- ties across the short stratigraphic intervals of anomalous inclinations and cause reports of questionable magnetic excursions abound in paleomag- immediately adjacent strata suggest that the magnetic mineralogy is uni- netic literature (for discussions, see Verosub and Baneijee, 1977; Bucha, form within these strata. The smaller remanent intensities observed in the 1983), careful consideration was given to attributing anomalous results anomalous samples may result from incompletely removed overprints of from the Tulelake record to past behavior of the geomagnetic field. After opposite polarity, or from reduced coherence of detrital remanence be- assessing the stability of magnetizations, demagnetization behavior, suscep- cause of diminished intensity of the geomagnetic field during excursions tibility, and magnetic fabric of multiple adjacent samples that yielded and polarity transitions. similar results, and considering the stratigraphic reproducibility from dif- Several complex and interrelated factors may have caused variation ferent cores that intersected the same horizons, we found no other plausi- in magnetic mineralogy in the Tulelake cores. These include changes in ble explanation for our anomalous results. Good tephrochronologic age provenance due to undetermined tectonic and volcanic events; geochemi- control exists within a few meters of all but one of these anomalous cal changes in depositional and diagenetic environment; and changes in intervals, and it strongly supports our proposed correlations with pre- climate that affected weathering, erosion, and transport of sediment into viously documented excursions. the basin. Susceptibility variation is much more pronounced in the upper third of the section than below about 100-m depth. This may indicate that Interval 1—Biwa?, 18 to 17 ka a diagenetic equilibrium is being approached in the older sediments, or alternatively, that source material and depositional conditions were indeed The youngest possible excursion in the Tulelake record lies between more varied for the upper interval. The susceptibility data provide an 5.27 and 5.53 m of depth. Here two adjacent samples (container numbers additional method for correlating horizons in these cores with future cores 164 and 165') from one core displayed negative remanent inclinations drilled in the Tulelake basin or other related lake basins of the region, and (Fig. 6) and primary detrital fabrics. Interpolation, assuming linear ac- possibly with regional or global records of paleoclimate. cumulation between the basal airfall pumice at Llao Rock (7.0 ka, Bacon, 1983) at about 2.2 m and the Trego Hot Springs Bed (23.4 ka; Davis, INTERPRETATION OF POLARITY AND CORRELATION 1978,1983) at 7.2 m, provides an estimated age of about 17 to 18 ka for this horizon. Note, however, that the ash layers were deposited during Polarity zonation of the Tulelake section is shown together with different climatic regimes (that is, different marine oxygen-isotope stages; inclination, intensity, and susceptibility data, in Figure 6. Correlation Fig. 7b), and thus sedimentation rates probably varied with changes in with the polarity time scale is confirmed down into the lower part of the climate. No correction for compaction with depth is attempted to offset Matuyama Chronozone (Figs. 6, 7a) by identification of five key tephra any false appearance of an accelerated accumulation rate toward the top of layers that have been independently dated from outside the Tulelake Basin. the cores. Identifications of the underlying Gauss Chronozone and the Kaena(?) We provisionally correlate this interval of anomalous inclinations in Subchronozone are based on their similarity in sequence and proportion to the Tulelake record with an excursion reported from Lake Biwa, Japan, at the corresponding intervals of the polarity time scale. Their correlation is about 18 ka (Kawai and others, 1972; Nakajima and others, 1973; Yas- consistent with apparent accumulation rates determined from the overly- kawa and others, 1973, p. 452; Kawai, 1984). Other possible correlative ing strata. No gaps long enough to bring older chronozones of similar magnetic events include the loosely dated Maelifell excursion of "Wiscon- proportion into the Tulelake sequence are indicated. Depths of the major sin age" (Pierce and Clark, 1978) and reports of various indications of chronozone, subchronozone, and excursion boundaries mentioned in the magnetic perturbations reported by Bucha (1983), Yaskawa (1974), and text are indicated. The age of sediments at the base of Tulelake core 2, as other workers. estimated by extrapolation from age control above, appears to be about 3.03 Ma. Paleomagnetic samples from 311 m to 330.48 m, however, 'All samples from the Tulelake cores have been assigned two numbers: an define an unbottomed zone of reversed polarity assigned to the Kaena(?) archival sample number and a container number. Container numbers are shown for Reversed-Polarity Subchron, radiometrically estimated to have begun paleomagnetic samples cited in the text.

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-BRUNHES- -MATUYAMA- -GAUSS- REUNION JARAMIU-0 OLDUVAI 2 1 KAENAt?) I I - J. V 0.8 2.0 2.4 2.8 3.2 -A1 -M1 -M

YA6

I A7

„A10 A9 -A11 A12 Figure 7a. Sedimentation-rate curve for the Tulelake core. Circles kM3 represent tephra layers (A1 through A25). Triangles represent mag- hA13 netozone boundaries (Ml through M9). Solid symbols indicate hori- *M4 lM5 zons for which independent age control is obtained from sites outside Tulelake. Open symbols represent tephra layers for which independ- A14 .A15 ent age control is not available; ages of these layers are estimated from • M6 the sedimentation rate curve.

_M7 S 160- k A16

\ A17

kA18 MS A19- A20 .M10 A21 „ A22 M11 » M12

, A23

5A25

Base of core

AGE, ka

100 200 300 400 500 600 700

Figure 7b. Detail of sedimentation-rate curve for the Tulelake core during the Brunhes Normal-Polarity 20 • Chron (0 to 83-m depth). Symbols and numbers for magnetostratigraphic and tephrochronologic datum levels are the same as for Figure 7a. Ages of marine oxygen-isotope stage boundaries are calibrated using £ 40 ah age of 0.75 Ma for the Brunhes/Matuyama Chron boundary (Sarna-Wojcicki and others, 1987), and as- X sumed constant sedimentation rate for core V29-238 of H a. Shackleton and Opdyke (1973) (see text). u Q 60 -

80 •

a b c a b c 6 7

OXYGEN ISOTOPE STAGES

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Explanation for Figures 7a and 7b

Tephrochronologic Datum Levels

AI — Basal airfall pumice at Llao Rock vent, ca. 7 ka (Bacon, A18 — ca. 1.97 Ma 1983) A19 — ca. 2.01 Ma A2 — Trego Hot Springs Bed of Davis (1978), ca. 23.4 ka A20 — ca. 2.02 Ma A3 — Olema ash bed, ca. 55 to 75 ka (Sarna-Wojcicki and others, A21 — ca. 2.05 Ma Ages estimated from sedimentation rate 1988; and this report) curve in Tulelake core. All tephra A4 — Tephra layer of sample T2382, "Pumice Castle-like tephra layers are silicic except A23 and A24, 2" layer, ca. 120 ka (based on stratigraphic position relative which are basaltic. to position of putative Blake Reversed-Polarity Subchron A22 — ca. 2.31 Ma [of Brunhes Normal-Polarity Chron]) and age constraints A23 — ca. 2.67 Ma at Crater Lake (Bacon, 1983) A24 — ca. 2.69 Ma A5 — Tephra layer of sample T64, tephra layer correlated to A25 — ca. 2.95 Ma DSDP Hole 173,120 to 130 ka, estimated from sedimentation-rate curve in Tulelake core A6 — Tephra layer of sample T1193, ca. 140 ka, estimated from sedimentation rate curve in Tulelake core A7 — Tephra layer of sample T2080L, ca. 155 to 160 ka, esti- mated from sedimentation rate curve in Tulelake core A8 — Tephra layer of sample T2023, ca. 160 ± 25 ka, estimated Magnetostratigraphic Datum Levels from K-Ar ages of units underlying and overlying correla- tive andesite ashflow tuff at Medicine Lake Volcano (J. M. (Age assignments from Mankinen and Dalrymple, 1979, Donnelly-Nolan and L. B. Pickthorn, 1989, written except for M1-M3) commun.) A9 — Rockland ash bed, 410 ka (Sarna-Wojcicki and others, Ml — Mono Lake(?) excursion, ca. 27 to 28 ka (Lund and others, 1985; C. G. Meyer, A. M. Sarna-Wojcicki, unpub. data) 1988) AIO — Dibekulewe Bed of Davis (1978), age between 410 and 610 M2 — Blake(?) Reversed-Polarity Subchronozone, ca. 114 ka ka, probably ca. 610 ka, estimated on basis of stratigraphic M3 — Base of Brunhes Normal-Polarity Chronozone, ca. 750 ka relationship to Lava Creek-B and Rockland ash beds, and (Sarna-Wojcicki and others, 1987; Izett and others, 1988) sedimentation-rate curve in Tulelake core M4 — Top of Jaramillo Normal-Polarity Subchronozone, All — Lava Creek-B ash bed, 620 ka (Izett and Wilcox, 1982) 900 ka A12 — Rye Patch Dam Bed of Davis (1978), ca. 630 ka, estimated M5 — Base of Jaramillo Normal-Polarity Subchronozone, from sedimentation-rate curve in Tulelake core 970 ka A13 — Tephra layer of sample T432, ca. 800 ka, estimated from M6 — Top of Olduvai Normal-Polarity Subchronozone, sedimentation-rate curve in Tulelake core 1.67 Ma A14 — Tephra layer of sample T542, ca. 1.35 Ma, estimated from M7 — Base of Olduvai Normal-Polarity Subchronozone, sedimentation-rate curve in Tulelake core 1.87 Ma Al 5 — Rio Dell(?) ash bed of Sarna-Wojcicki and others (1982) M8 — Top of Reunion 2(?) Normal-Polarity Subchronozone, (of the Rio Dell Formation of Ogle, 1953), ca. 1.45 Ma 2.01 Ma (Sarna-Wojcicki and others, 1987); tephra layer of sample M9 — Base of Reunion 2(?) Normal-Polarity Subchronozone, T558 2.04 Ma A16 — T682, ca. 1.9 Ma estimated from sedimentation-rate curve M10 — Top of Reunion 1(?) Normal-Polarity Subchronozone, in Tulelake core 2.12 Ma A17 — Two thick sequences of coarse tephra layers, samples T693 Mil — Base of Reunion 1(?) Normal-Polarity Subchronozone, through T712, and T715 through T726, and Bear Gulch 2.14 Ma ash bed of Sarna-Wojcicki and others (1982) (of the Rio M12 — Base of Matuyama Reversed-Polarity Chronozone, Dell Formation of Ogle, 1953), ca. 1.91 to 94 Ma, esti- 2.48 Ma mated from sedimentation-rate curve in Tulelake core and M13 — Top of Kaena(?) Reversed-Polarity Subchronozone, from Centerville Beach section, Humboldt Co. 2.92 Ma

Interval 2—Mono Lake?, 28 to 27 ka Bed and the Olema ash bed (55 ka to 75 ka; Sarna-Wojcicki and others, 1991) at a depth of 13.0 m gives an estimated age of between about 33 ka Between 9.1 and 10.51 m of depth, five of six samples (container and 53 ka for this interval. numbers 16, 230, 231, 232, 233, 234) show negative or very shallow This age estimate must, however, be used with caution because the positive remanent inclinations (Figs. 5,6) and primary detrital fabrics (Fig. two tephra layers are found in strata deposited during contrasting oxygen- 5). These samples were taken from two separate but stratigraphically isotope stages (Fig. 7b), and sedimentation rates probably varied consider- overlapping cores. One core segment yielded both positive and negative ably with changes in climate. Furthermore, no period of anomalous inclinations. Straight-line interpolation between the Trego Hot Springs magnetization between 8,000- and 13,000-yr duration within this time

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interval, as implied by the straight-line accumulation rate, is likely to have others, 1988). Age of the Mono Lake excursion is between 28 ka and 27 escaped previous notice. In all probability, the sedimentation rate was ka (Lund and others, 1988). particularly high when these strata were deposited. Although uncertainties in declination of the Tulelake core segments Late Wisconsinan climate changes are likely to have affected regional preclude calculation of virtual geomagnetic pole (VGP) positions through erosion and sedimentation within the Tulelake basin and to have caused this anomalous interval, other paleomagnetic characteristics support this relatively rapid sedimentation from about 28 ka to 10 ka (oxygen-isotope correlation. The negative inclinations in the Tulelake record are of similar stage 2). Abundant lake sediments of late Wisconsinan age in the western magnitude to those of nearby records of the Mono Lake excursion, and the United States suggest that active sedimentation was regionally widespread. pattern of changes in remanent intensities through the interval, unusually Somewhat slower sedimentation probably took place during the period low in samples of negative inclination and unusually high near the end of between about 60 ka and 30 ka. the excursion, is consistent with other records. Overly steep inclinations Our inferences about changes in sedimentation rate, and our identifi- near the end of the Mono Lake excursion, reported from some other cation of an ash layer associated with the Mono Lake excursion at records (Liddicoat and Coe, 1979; Lund and others, 1988), are not seen in Summer Lake, Oregon, within this interval (see discussion of ash layer the Tulelake data. T2438, below), have led us to propose that this interval of anomalous Other reported magnetic events between eruption of the two tephra inclinations in the Tulelake cores is correlative with the Mono Lake excur- layers used here for age control include the Laschamp Reversed-Polarity sion reported from late Wisconsinan-age sediments of several related lake Subchron (Bonhommet and Zahringer, 1969), variously estimated as basins in the western United States (Denham and Cox, 1971; Denham, being between about 40 ka and 50 ka (for example, Gillot and others, 1974; Liddicoat and Coe, 1979; Negrini and others, 1984; Lund and 1979; Hall and York, 1978); the Lake Mungo excursion, estimated as

TABLE 1. ELECTRON-MICROPROBE ANALYSIS OF VOLCANIC GLASS SHARDS FROM 17 MAJOR TEPHRA LAYERS IN CORES FROM TULELAKE, CALIFORNIA. AND CHEMICALLY CORRELATIVE OR CHEMICALLY SIMILAR TEPHRA LAYERS FROM LOCALITIES OUTSIDE OF TULELAKE

Sample no. Si02 Al2Oj Fe203 MgO MnO CaO TiOz NazO K20 T0

Tephra layer at 1.73-2.28 m in Tulelake cores, average of 4 samples (1); basal pumice at Llao Rock, Crater Lake, OR (2); and Tsoyowata Bed of Davis (1978) at Swamp Lake, Sierra Nevada, CA (3)

1. 74.2 14.3 1.84 0.30 0.06 1.21 0.32 4.9 2.96 97.8 ±s 0.1 0.1 0.03 0.02 0.01 0.04 0.02 0.2 0.04 0.3 2. 74.1 14.3 1.79 0.30 0.05 1.21 0.31 5.1 2.91 ND 3. 74.1 14.3 1.86 0.31 0.04 1.22 0.29 4.9 2.95

Tephra layer at 7.16-7.23 m in Tulelake cores, average of 6 samples (4); Redcloud airfall pumice layer at Crater Lake, OR (5); and Trego Hot Springs tephra layer of Davis (1985), at Summer Lake, OR (6)

4. 75.5 13.6 1.58 0.21 0.05 1.02 0.23 4.5 3.32 95.3 ±s 0.2 0.1 0.04 0.01 0.01 0.03 0.01 0.1 0.06 1.0 5. 74.9 13.7 1.67 0.24 0.04 1.11 0.24 4.8 3.37 96.7 6. 75.7 13.6 1.58 0.20 0.03 1.04 0.22 4.5 3.21 ND

Tephra layer at 9.34 m in Tulelake core (7), and tephra bed El of Davis (1985) at Summer Lake, OR (8)

7. 76.8 13.0 1.33 0.17 0.04 0.80 0.20 4.2 3.52 94.72 8. 76.7 13.1 1.25 0.14 0.04 0.75 0.18 4.2 3.61 ND

Tephra layer in nearehore lake beds at Tulelake, near Newell (9); and Marble Bluff tephra layer of Davis (1985), at Summer Lake, OR (10), equivalent to Mount St. Helens Cw bed

9. 76.9 13.8 0.93 0.21 0.04 1.51 0.11 4.1 2.44 94.20 10. 76.7 13.9 0.91 0.23 0.04 1.50 0.10 4.2 2.41 ND

Tephra layer at 13.04 m in Tulelake core (11); and Olema ash bed of Sama-Wojcicki and others (1988), Clear Lake, CA (12)

11. 75.2 13.3 1.74 0.10 0.07 0.49 0.21 4.8 4.08 94.6 12. 75.3 13.4 1.75 0.11 0.06 0.48 0.17 4.6 4.08 94.0

Tephra layer at 16.99 m in Tulelake core (13); airfall pumice at Cloudcap Road, Crater Lake, OR (14); and tephra layer 6 of Davis (1985), at Summer Lake, OR (15)

13. 72.2 14.5 2.63 0.49 0.05 1.73 0.47 4.6 3.31 96.7 14. 72.3 14.3 2.51 0.51 0.06 1.74 0.45 4.9 3.22 96.8 15. 72.4 14.7 2.38 0.49 0.03 1.77 0.49 4.7 3.01 ND

Tephra layer at 17.01 m in Tulelake core (16); welded airfall pumice east of Llao Rock, of Bacon (1983); and disseminated tephra at 3.7 and 4.6 m in DSDP Hole 173, in the northeastern Pacific Ocean (18, 19)

16. 70.6 14.7 3.16 0.72 0.08 2.34 0.65 4.9 2.92 96.0 17. 70.7 14.9 3.22 0.71, 0.06 2.27 0.69 4.6 2.91 ND 18. 70.4 15.0 3.14 0.68 0.08 2.25 0.66 4.8 2.93 98.7 19. 70.6 14.9 3.09 0.70 0.04 2.29 0.62 4.9 2.92 98.4

Tephra layer at 32.28 m in Tulelake core at 32.28 m in Tulelake core (20); and tephra layer V of Davis (1985) at Summer Lake, OR (21)

20. .. 73.34 14.22 2.61 0.15 0.07 1.11 0.22 4.8 3.47 94.25 21. 72.84 14.23 2.68 0.17 0.06 1.07 0.23 5.2 3.51 ND

Heterogenous tephra layer at 53.07-53.13 m in Tulelake core, average of 2 samples (22); tephra layer KK of Davis (1985) at Summer Lake, OR (23); and heterogenous andesitic ash flow tuff, intermediate iron fraction, average of 2 samples, at Medicine Lake Volcano (24)

22 63.0 16.2 6.07 2.19 0.12 4.78 0.98 4.6 2.09 97.4 23. 62.8 16.3 6.19 1.98 0.11 4.73 0.98 4.8 2.11 ND 24. 64.9 16.1 5.24 1.65 0.09 4.01 0.91 4.6 2.54 ND

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being about 30 ka (Barbetti and McElhinny, 1972, 1976); the Karga- Crater Lake, Oregon (estimated between 72 ka and 117 ka in age; Bacon, polova excursion, estimated at about 40 ka (Bucha, 1983); an excursion at 1983) (Tables 1, 2, and A)l about 50 ka reported from Skalamaelifell, Iceland (Levi and others, 1987); Linear interpolations between the Olema ash bed above and tephra and an excursion reported from Lake Biwa at about 49 ka (Yaskawa and sample T2382 below, using the stratigraphically more reasonable age es- others, 1973, p. 452; Nakajima and others, 1973). timates for their overlapping ranges, provide age estimates between about 105 ka and 114 ka for this horizon. Thus, we provisionally correlate these Interval 3—Blake?, 112 to 114 ka negative paleomagnetic inclinations with the Blake Reversed-Polarity Subchron. The Blake "event" is recognized in numerous other studies. Between depths of 16.0 and 16.7 m, two samples (container numbers Estimates of its age range from 70 ka to 160 ka, with most falling between 20, 242) from separate cores show negative inclinations and undisturbed 100 ka and 130 ka (Champion and others, 1988, for discussion). Its age detrital fabrics. We provisionally correlate this horizon with the Blake Reversed-Polarity Subchron. Approximate age control for this interval at Tulelake is provided by the Olema ash bed, and by tephra sample T2382 2Table A may be secured free of charge by requesting Supplementary Data from 16.99 m, which correlates with an air-fall pumice at Cloudcap Road, 9209 from the GSA Documents Secretary.

TABLE 1. (Continued)

Sample no. Si02 Al203 Fe203 MgO MnO CaO Ti02 Na20 K20 T„

Tephra layer at 59.75-59.76 m in Tulelake core, average of 3 samples (25). and ash 7 at 148 m in core (Sarna-Wojcicki and others, 1988), at Clear Lake, CA (26)

25. 76.7 12.9 1.05 0.06 0.04 0.47 0.09 3.8 4.93 94.1 26. 76.8 12.8 1.07 0.07 0.05 0.45 0.06 3.8 4.83 95.0

Tephra layer at 59.54-59.99 m in Tulelake core, average of 7 samples (27); Rockland ash bed of Sarna-Wojcicki and others (1985), at Lassen Peak area, CA (28); Rockland ash bed at Oreana, NV (J. O. Davis, 1985, personal commun.) (29); and Rockland ash bed in Humboldt Basin, CA (29)

27. 78.1 12.5 0.86 0.17 0.03 0.85 0.16 3.9 3.52 93.6 ts 0.1 0.1 0.01 0.01 0.01 0.01 0.01 0.1 0.03 0.4 28. 78.0 12.5 0.86 0.18 0.03 0.85 0.16 3.8 3.69 95.2 29. 77.8 12.7 0.89 0.19 0.04 0.86 0.18 3.9 3.51 ND 30. 78.0 12.5 0.85 0.19 0.04 0.85 0.16 3.8 3.59 94.1

Tephra layer at 62.05-62.27 m in Tulelake cores, average of 6 samples (31); and Dibekulewe Bed of Davis (1978) in Lahontan Basin, near Carson City, NV (32)

31. 76.9 12.9 1.20 0.03 0.04 0.55 0.06 4.2 4.14 93.9 ±s 0.3 0.3 0.10 0.02 0.01 0.06 0.02 0.1 0.14 0.6 32. 76.6 13.1 1.30 0.05 0.03 0.61 0.07 4.1 4.11 ND

Tephra layer at 64.59 m in Tulelake core, average of 2 samples (33); Lava Creek B ash bed at Oreana, NV (Davis, 1978) (34); and Lava Creek B ash bed at Onion Creek, UT (Izett, 1981; Sarna-Wojcicki and others, 1984) (35)

33. 76.8 12.2 1.60 0.03 0.04 0.53 0.11 3.5 5.14 94.9 34. 77.0 12.3 1.57 0.02 0.05 0.53 0.11 3.4 4.99 ND 35. 77.0 12.1 1.55 0.03 0.04 0.52 0.10 3.7 4.99 ND

Tephra layer at 65.38 m in Tulelake core (36); and Rye Patch Dam Bed of Davis (1978), at Oreana, NV (37)

36. 70.9 14.8 3.36 0.35 0.08 1.42 0.43 5.2 3.50 94.4 37. 71.3 14.8 3.22 0.34 0.06 1.39 0.43 5.0 3.49 94.8

Tephra layer at 130.36 m in Tulelake core (38); Rio Dell ash bed in Humboldt Basin, CA (Sama-Wojcicki and others. 1987) (39); and Rio Dell ash bed at 23 m in DSDP Hole 34, northeastern Pacific Ocean (40)

38. 73.6 14.7 1.87 0.34 0.08 1.31 0.31 4.9 2.86 93.8 39. 74.1 14.5 1.92 0.28 0.08 1.35 0.24 5.3 2.36 94.1 40. 73.9 14.5 1.87 0.28 0.07 1.32 0.29 5.5 2.32 93.5

Suite of coarse, 7-m-thick, heterogenous tephra layers at 173.80 to 181.20 m, in Tulelake core; compositional range of 12 samples (41 to 42); tephra layer at 176.94 m in Tulelake core (43); and similar Bear Gulch ash bed in Rio Dell Formation of Humboldt Basin, CA (44)

41 70.3- 14.6- 2.22- 0.39 0.05 1.34 0.34- 4.1- 3.62- 86.1 42. 73.1 15.3 2.82 0.77 0.08 2.20 0.49 4.9 4.04 94.4 43. 71.5 14.9 2.41 0.55 0.05 1.64 0.41 4.6 3.93 94.1 44. 72.1 14.8 2.35 0.44 0.03 1.56 0.39 5.1 3.17 94.2

Tephra layer at 314.96 m in Tulelake core, average of 2 samples (45); and tephra layer at Chalk Bank Landing, Lower Klamath Lake, CA (46)

45. 73.2 14.6 2.11 0.18 0.07 0.88 0.22 5.1 3.71 92.9 46. 73.4 14.4 2.13 0.20 0.07 0.88 0.26 4.8 3.86 92.0

Homogenous, natural glass standard, RLS 132; average of 18 analyses

47. 75.4 11.3 2.12» 0.06 0.16 0.11 0.19 4.9 4.42 98.6 ±s 0.1 0.2 0.04 0.01 0.01 0.01 0.01 0.1 0.06 ND

Note: values given for the oxides are in weight percent, recalculated to a 100% fluid-free basis. TQ, original total before recalculation, where available. See Table A, available free of charge from the Society's data repository, for electron-probe analysis of all 128 tephra samples from Tulelake cores. ±s, one standard deviation calculated for four or more replicate samples. ND, not available. Multiple analyses of a homogenous natural glass standard, RLS 132, are given at the bottom of the table, and provide a close estimate of the analytical error for each oxide in electron-probe analysis. C. E. Meyer, U.S. Geological Survey, Menlo Park, analyst. 'Iron reported as FeO for the standard.

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TABLE 2. CORRELATION OF TEPHRA LAYERS AT TULELAKE, CALIFORNIA (LOCALITIES 4, 5), WITH TEPHRA LAYERS AT OTHER LOCALITIES IN THE WESTERN UNITED STATES

I 2 3 4 5 6 7 8 9 10 11 Name(s) of Mount St. Crater Summer Tulelake, Tulelake, Lassen Lahontan DSDP Hole Humboldt Clear Onion Age of Eruptive source References to age. tephra Helens, Lake, Lake, CA lower Peak Basin, 173, north- Basin, Uke, Creek, tephra of tephra stratigraphy, and layers) WA OR OR (cores) Klamath area, NV east Pacific CA CA UT layer layer other information (COMP.) Basin, CA CA (COMP.) Ocean (COMP.) (core) (COMP.) (core)

T1163, T6, etc. LD-28, etc. Tsoyowata bed, 7015 yr B P, 14C Crater Lake, OR Davis, 1978; Bacon (1983)» b3sal pumice at Llao Rock Vent CRL-10 GS-53 T175 CB-33 etc. T176, etc. 61285-33 LT-2, etc.; "C Trego Hot Springs bed, 23.4 ka, 14C Crater Lake, OR Davis, 1983«, 1985 Redcloud pumice layer DR-82; * T2439; " Summer Lake tephra 26-27 ka, " Unknown Davis, 1985; this report* bed El

CwSH(l); l4C Marble Bluff bed. 33.4; 37.6 ka, l4C Mount St. Helens, Davis, 1978-, 1985; Mulli- Mount SL Helens WA neaux, 1986* setC CL0172; C, A 55-75 ka, 14C, Unknown Sarna-Wojcicki and others, CRL-9; A,# 1988*; this report* CB-7, etc. GS-57 K-Ar GS-58 T2383; " Airfall pumice at 7 120 ka, K-Ar,# Crater Lake, OR Bacon, 1983*; this report* Cloudcap Road, "Pumice-Castle- like tephra 2" DSDP-173- T64; 1-3,4;" DSDP-173-1-3,4 120-130 ka," Crater Uke, Sama-Wojcicki and others, (160-180 ka,") OR (?) 1987*;this report* DR-24 (70 ka, K-Ar) Bacon, 1983* DR-25 Summer Lake tephra 135-140 ka,* Unknown Davis, 1985: this report* bed V

DR-33 T2023 190M; K-Ar Andesitic ash-flow 160 ± 25 ka. Medicine Lake J. M. Donnelly-Nolan and tuff of Medicine K-Ar;" Volcano, CA LB. Pickthorn (written Lake Volcano; commun, 1989)*; this Summer Lake report* tephra bed KK

Clear Lake tephra 0.41 Ma, * Medicine Lake Sama-Wojcicki and others. layer CL0590 Volcano, CA (?) 1988*; this report* T1245, RPT(L)3; DR-17 T291, etc. K-Ar etc. Rockland ash bed; 0.41 Ma, FT, Lassen Peak area, Sama-Wojcicki and others, Rockland pumice K-Ar CA 1985*; C. E. Meyer and tuff breccia of J. O. Davis, A. M. Sarna-Wojcicki, Wilson, I960 written unpub. data* commun., T1249.T319, 1987 etc.;* Dibekulewe bed 0.61 Ma," Unknown Davis, 1978;this report*

ORNA-1, LD-71, etc. 67W104; FT Lava Creek B ash bed; 0.62 Ma, K-Ar, Yellowstone Izett, 1981* Pearletle FT National Park, type O ash bed LD-61, WY, ID LAH-2 Rye Patch Dam bed 0.63 Ma, * Central Cascade Davis, 1978; this report* Range, OR (?) DSDP-173- RD-2

Rio Dell ash bed; 1.45 Ma, FT, p Central Cascade Izat, 1981*; Sama-Wojcicki Centerville Beach Range, OR (?) and others, 1987s ash bed T693 through T726; * SM-ASM-7; " 1.92-1.94 Ma, " S. Cascade Range, This report* Medicine Lake Highland, CA, T1066t OR T1893t 61284-30 Tephra layer of 7 2.95 Ma, » Unknown This report* sample T1893

Note: (COMP.), composite section from two or more sites within a limited area; age, numerical age by: 14C - radiocarbon date; A, age estimate based on amino-acid racemization stereochemistry; FT, fission-track age; K-Ar, potassium-argon age;age estimate from sedimentation-rate curve based on interpolation between magnetostratigraphic, tephrochronologic, btostratigrapbic, or other age data; »reference to numerical age conrol of a particular tephra layer; other references are to names, stratigraphy, or other information on specified tephra layer; etc, multiple samples from a particular site were analyzed; additional sample numbers are not given due to space limitations. ^Indicates numerical age control is obtained at this site for tephra bed.

was estimated at about 112 ka to 114 ka by Smith and Foster (1969) and inclinations and undisturbed fabrics. We estimate the age of this short Denham and others (1977). It occurred during marine oxygen-isotope interval to be between about 150 and 155 ka, based on interpolation stage 5E. between tephra layers at 16.99 m, 17.01 m, and 53.07 m of depth in the Tulelake section. Those tephra layers correlate with ash-flow tuffs that are Interval 4—150 to 155? ka constrained in age between about 117 ka and about 180 ka. The Jamaica Between 45.4 and 45.6 m of depth, two samples from one core Reversed-Polarity Subchron occurred early within this time interval (container numbers 181, 183), show negative or very shallow positive (Champion and others, 1988), but age control in the Tulelake cores is

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insufficient to propose a correlation. Previous estimates for this interval of well with an airfall pumice at Llao Rock vent, Crater Lake, Oregon. In the Tulelake cores, using straight-line interpolation between the Rockland Tulelake core 1, this tephra layer is found at a nominal depth of 2.28 m ash bed below and tephra sample T2382 above, provided an age estimate (sample T6), where it is about 7 cm thick. Two chemically similar tephra of about 310 ka for this horizon. We now believe, however, that earlier layers, each 4 cm thick, are also found in core 1 at depths of 2.42 m estimate to be in error (Sarna-Wojcicki and others, 1991). (sample T2244) and 2.75 m (sample T2245). These layers from core 1 are all chemically similar to the proximal basal airfall pumice at Llao Rock Interval 5—Kamikatsura?, 850 ka vent (Bacon, 1983; sample CB-34 of J. O. Davis, 1986, written commun.; similarity coefficients (S.C.) range from 0.97 to 0.99). Radiocarbon analy- Two short intervals of positive inclination and undisturbed detrital sis dates the basal airfall pumice at Llao Rock at 7015 yr B.P. (Bacon, fabric are found within the upper part of the Matuyama Reversed-Polarity 1983). Chronozone. The upper interval—between 88 and 89 m of depth—is The 8-cm-thick tephra layer at 1.77 m in Tulelake core 4 and the recorded in three samples (container numbers 69, 284, 285) from two 7-cm-thick layer at 2.28 m in core 1 are probably the same layer because separate but stratigraphically overlapping cores. Interpolation between the they are similar in both thickness and chemical composition. Differences Matuyama-Brunhes boundary (considered herein to be 750 ka; Sarna- in depths between cores 1 and 4 suggest that a misregistration of about Wojcicki and others, 1987) at 82.6 m of depth and the top of the Jaramillo one-half meter exists between these two cores at this horizon. The tephra Normal-Polarity Subchronozone (900 ka; Mankinen and Dalrymple, layer at 2.42 m in core 1 correlates best with an undated airfall pumice 1979) at 101.8 m of depth, indicates an age of about 800 ka for these layer at Crater Lake (CRL-1; S.C. = 0.99) that overlies breccia formed by samples. This interval may represent the Kamikatsura Normal-Polarity the collapse of Munson dome (Bacon, 1983) and also almost as well with Subchron, estimated at about 800 ka by Maenaka (1980). Other reports of the basal airfall pumice at Llao Rock vent (S.C. = 0.98). The age of this normal polarity of similar age include volcanic rock radiometrically dated layer estimated from the sedimentation-rate curve for Tulelake (Fig. 7b) is at about 840 ka (Mankinen and others, 1981), and an event recognized in 7.6 ka. Layers at 2.75 m in core 1,2.28 m in core 1, and 1.77 m in core 4 marine magnetic anomaly data originally estimated at about 820 ka (Wat- are chemically indistinguishable on the basis of EMA from the basal airfall kins, 1968) and since revised to about 850 ka, using more recent time pumice at Llao Rock vent (S.C. = 0.98). Using the Tulelake scales (Champion, 1988). sedimentation-rate curve (Fig. 7b), we estimate the age of the ash layer at 2.75 m in core 1 to be about 8.3 to 8.4 ka. Interval 6—Cobb Mountain?, 1.10 Ma The basal airfall pumice at Llao Rock vent correlates with the distal Tsoyawata bed of Davis (1978, 1985). This ash bed has been found at a The lower interval of anomalous inclinations within the Matuyama number of localities to the east and southeast of Crater Lake, including the Reversed-Polarity Chronozone lies between 111.71 and 113.59 m of Lahontan basin (Davis, 1978). At many localities, this bed closely under- depth and is represented by three consecutive samples (container numbers lies the climactic (informal) Mazama ash bed (Davis, 1978; Bacon, 1983), 308, 309, 310) that display positive inclinations and primary detrital fab- but at Tulelake and several sites farther to the south and southeast, the rics. Interpolation between the base of the Jaramillo Normal-Polarity Mazama ash bed is absent. Subchronozone (970 ka; Mankinen and Dalrymple, 1979) at 106.2 m of depth and the top of the Olduvai Normal-Polarity Subchronozone (1.67 Trego Hot Springs Bed Ma; Mankinen and Dalrymple, 1979) at 138.8 m of depth gives an esti- mated age of 1.09 to 1.13 Ma for these samples. This age supports correla- A 3-cm-thick tephra layer at 7.23 m (samples T175, T176, T2307, tion with the Cobb Mountain Normal-Polarity Subchron estimated at 1.12 and T2308; Table A, Table 2; Figs. 6,7b) in Tulelake core 2, matches well Ma by Mankinen and others (1978), at 1.10 Ma by Mankinen and with the Trego Hot Springs Bed of Davis (1978) (S.C. = 0.98-0.99). Based Gromme (1982), and between 1.11 Ma and 1.10 Ma by Hsu and others on upward extrapolation from radiocarbon dates underlying the Trego (1990). These ages are also in general agreement with estimates for excur- Hot Springs Bed at Pyramid Lake, Nevada, Davis (1983) estimated its age sions proposed by Maenaka (1980) and for sea-floor anomalies reported to be 23.4 ka. The same tephra layer is also found in nearshore sands and by Rea and Blakely (1975). gravels at the southeastern margin of Tulelake, near Newell (sample 61284-2), where it overlies the Mount St. Helens C ash bed (sample IDENTIFICATION AND CORRELATION OF 61585-52C; Table A). The Trego Hot Springs Bed has been identified INDIVIDUAL TEPHRA LAYERS from sites in western Nevada, northeastern California, and southeastern Oregon, including Summer Lake, Oregon (Davis, 1985). Davis (1978) Samples (127) from 63 tephra layers in the Tulelake cores have been suggested that this tephra was erupted from Crater Lake, Oregon, because analyzed. Chemical analyses of these tephra layers are presented in Table of its chemical similarity to other tephra from that source. We confirm A3; an abbreviated version summarizing results from 47 layers is presented Davis' (1978) suggestion and propose that the near-source exposures of in Table 1; and regional correlations for 17 of the layers are shown in the Trego Hot Springs Bed are equivalent to the Redcloud pumice airfall Table 2. Identified tephra layers that have been independently dated or layer of Bacon (1983) at Crater Lake (sample CRL-10, and sample CB-33 observed in stratigraphic context at localities outside the Tulelake basin are of J. O. Davis, 1986, written commun.) (Table 2; Table A; S.C. 0.98). described below. Many of the tephrostratigraphic correlations are made on the basis of EMA alone and should be considered provisional. Future Tephra Layer of Sample T2438 and Summer Lake Tephra Bed analyses by more sensitive techniques such as XRF and neutron activation El of Davis (1985) may refine these correlations. A thin tephra layer at a depth of 9.34 m in Tulelake core 2 (sample Basal Airfall Pumice at Llao Rock Vent; Tsoyawata Ash Bed T2438; Table A, Table 2) chemically matches Summer Lake tephra bed El in upper Pleistocene lake beds near Summer Lake, Oregon (sample An 8-cm-thick tephra layer found at a depth4 of 1.77 m in Tulelake core 4 (samples T1163, T2309; Table A, Table 2; Figs. 7a, 7b), matches 4Depth of a tephra layer in the Tulelake cores is the position of the base of the 3See footnote 2. layer.

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DR-82 of Davis, 1985). The estimated age of sample T2438, based on the Tephra Layer of Sample T64, Disseminated Tephra at 3.7 and 4.6 m in Tulelake sedimentation rate curve, is 26 to 27 ka. At Tulelake, this tephra DSDP Hole 173, and Welded Airfall Pumice East of Llao Rock Vent layer is situated within the upper part of an ~1.4-m-thick zone of anoma- lous paleomagnetic inclinations that we correlate here with the Mono Sample T64, from an unnamed 3-cm-thick ash bed at 17.01 m in Lake magnetic excursion. The Mono Lake excursion is identified at Tulelake core 1, correlates well (S.C. = 0.98-0.99) with two zones of Summer Lake within a 25-cm interval between the Wono and Marble disseminated ash found at depths of 3.7 and 4.6 m in Deep Sea Drilling Bluff beds (Negrini and others, 1984); there, bed El lies about 25 cm Project (DSDP) Hole 173 in the northeast Pacific Ocean (Sarna-Wojcicki above the Wono bed. Neither the Wono nor the Marble Bluff (= Mount and others, 1987). The uppermost sediments from DSDP 173 are dis- St. Helens C) beds have been found in the Tulelake core. At Summer turbed, and the two zones of disseminated ash may represent a single bed, Lake, bed El is found about 10 cm below the Trego Hot Springs Bed. At repeated by transposition during coring. Biostratigraphic age control and Tulelake, sample T2438 (= bed El) underlies the Trego Hot Springs Bed assumed zero age at the sediment surface led Sarna-Wojcicki and others by about 2.1 m. Thus, the record of magnetic excursion at Tulelake may (1987) to give approximate age estimates of 160 ka and 180 ka for the two encompass a longer time period than the record at Summer Lake, or the ash layers in DSDP Hole 173. New data on the andesitic ash bed of Summer Lake stratigraphic record may be more compressed within this sample T2023, underlying sample T64 at Tulelake, however, indicate that time interval than that of Tulelake. sample T64 and its correlatives in DSDP Hole 173 are about 130 ka. This revised estimate is based on positions of the Blake Subchron and the Mount St. Helens C (Marble Bluff Bed of Davis, 1978,1985) andesitic ash of sample 2023 at about 53 m at Tulelake. Tephra layers T64 and DSDP-173-1-3,4 also match well (S.C. = The Mount St. Helens C layer has not been found in the Tulelake 0.98-0.99) with a welded airfall pumice in the caldera wall east of Llao cores, but it is present in outcrops of nearshore deposits at the late Pleisto- Rock vent, at Crater Lake (CB-12; J. C. Davis, 1986, written commun.; cene southeastern margin of Tulelake, near Newell (sample 61585-52C; Table 2). Bacon (1983), however, estimated the age of this unit to be about Tables A and 2). There, the layer underlies the Trego Hot Springs Bed and 70 ka (his sample 79C166), considerably younger than our estimate from overlies the Olema ash bed. Its age is estimated from related radiocarbon the Tulelake core. Consequently, this correlation is in doubt. ages to be between about 33.7 ka (Marble Bluff bed of Davis, 1985) and 36 to 37.6 ka (Mount St. Helens set C of Mullineaux, 1986). The tephra Tephra Layer of Sample T1193 layer from Tulelake at Newell resembles most closely layer Cw of Mulli- neaux's set C (1986, sample CwSHl; Table A). Sample T1193, from a tephra layer at a depth of 32.28 m in Tulelake core 5, correlates (S.C. = 0.97) with Summer Lake tephra bed V at Olema Ash Bed Summer Lake (sample DR-25) situated 8 m stratigraphically below the Trego Hot Springs Bed at the latter locality (Davis, 1985). We estimate the Spot sample T199, from a thin disseminated ash at 13.04 m in age of this ash bed to be about 140 ka, based on interpolation between the Tulelake core 2 (Tables A and 2), matches the (informal) Olema ash bed Blake Subchron above, and the age of an andesitic ash (sample T2023) found in cores from Clear Lake, west-central California. We estimate its lower in the core. age between 55 ka and 75 ka based on (1) downward extrapolation of sedimentation rates determined from radiocarbon ages at Clear Lake Tuffaceous Interval between Depths of about 50 and 60 m (Sarna-Wojcicki and others, 1988), (2) palynological correlations between sediments of Clear Lake and the deep-ocean oxygen-isotope record An interval dominated by multiple tephra layers and reworked (Adam, 1988), and (3) constraining age datums above and below at tephra in the Tulelake cores between 48,98 and 59.54 m of depth marks a Tulelake. Glass of this ash bed is homogeneous. Similarity coefficients period of increased volcanic activity, or a period of slow deposition. The between this ash bed at Tulelake and exposures elsewhere in California are tephra layers of airfall origin are homogeneous and coarse, suggesting that between 0.98 and 0.99. they were erupted from a nearby source, possibly Medicine Lake Volcano (Fig. 1). Some of these layers are chemically similar to those from the Airfall Pumice at Cloudcap Road (= "Pumice Castle-like 2" tephra) Mono Craters of east-central California (between about 600 yr and and tephra of Pumice Castle 100,000 yr old), but which do not correlate with the Tulelake layers because the latter are older, about 155,000 to 400,000 yr (Figs. 7a, 7b). Sample T2382 from a 6-cm-thick tephra layer at 16.99 m in Tulelake Other tephra layers from this interval (for example, samples T260, T2038, core 1 (Table A; Fig. 7b) matches well (S.C. = 0.97-0.98) with the airfall T2099L, and T275; Table A) do not match well with any other tephra pumice at Cloudcap Road (equivalent to "Pumice Castle-like tephra 2" of layers we have analyzed from outside the Tulelake basin. We believe that Davis, [1985]; S.C. = 0.99), and almost equally well with the tephra of these samples represent tuffaceous intervals within which tephra of two or Pumice Castle (Davis, 1985; samples CB-7, -8, -9; S.C. = 0.98), at Crater more ash layers are mixed. Lake. At Tulelake, this layer is a few centimeters below the putative Blake Reversed-Polarity Subchron. Our sedimentation-rate curve implies an age Tephra Layer of Sample T2080L of about 120 ka. At Crater Lake, the tephra of Pumice Castle and "Pumice Castle-like tephra 2" are bracketed between about 72 ka and 117 ka by Pumice lapilli collected from a depth of 50.32 m in the Tulelake core K-Ar ages on overlying and underlying flows (C. R. Bacon and M. A. are chemically similar to the heterogeneous Wadsworth ash bed, found Lanphere, 1985, written commun. to J. O. Davis), an age compatible with near the stratigraphic middle of the pluvial Eetza Formation in the Lahon- that proposed here. tan basin of Nevada (Davis, 1978,1985; Morrison and Davis, 1984). The

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Eetza Formation is thought to have been deposited during oxygen-isotope coat and Bailey, 1989). This excursion was provisionally identified as the stages 6 and 10 (Morrison and Davis, 1984). Our estimate of the age of Levantine, or Biwa II event, about 280 to 295 ka, based on previous ash bed T2080L, based on interpolation between the Blake Reversed- estimates provided to Liddicoat and Bailey by A. M. Sarna-Wojcicki. Polarity Subchron above and the andesitic ash of sample T2023 below, is These estimates now appear to be in error. Using revised age estimates of 155 ka—within stage 6. The eruptive source of sample T2080L is not ash beds in the Tulelake core, the magnetic event at Paoha Island and known, but the coarseness of the pumice lapilli (to 1 cm in diameter) in the Benton Crossing is more likely to be the Jamaica event, thought to have Tulelake core suggests that it may have been erupted from nearby Medi- occurred sometime between about 160 ka and 200 ka (Champion and cine Lake or Shasta Volcanoes in northern California, or from Crater Lake others, 1988). A single paleomagnetic sample from 51.95 m in Tulelake in southern Oregon. core 2 (Fig. 6) showed stable negative (reversed) inclinations (-40° to -46°) through all five demagnetization steps and may represent this event. Tephra Layer of Sample T2023: Andesitic Tuff of Medicine Lake Volcano, and Summer Lake Tephra Bed KK Rockland Ash Bed

The andesitic tephra of sample T2023, from 53.07 m depth in the The Rockland ash is found in a 47-cm-thick zone at depths of 59.42 Tulelake section, correlates well on the basis of shard chemistry with an to 59.89 m in Tulelake core 2. It occurs as several discrete beds: at 59.89 m andesitic ashflow tuff (sample 194M) exposed on the lower flanks and (sample T1403,5 cm thick), 59.69 m (sample T295, a thin layer identified within the crater of Medicine Lake Volcano, several tens of kilometers from a spot sample), at 59.59 m (T2122, 1 cm thick), and at 59.42 m southwest of Tulelake (Donnelly-Nolan and Nolan, 1986). The glass of (T2123, 12 cm thick, Table A). At other localities, there is evidence for this tephra layer is dacitic and heterogeneous; the whole-sample composi- only one airfall layer, usually overlain by locally reworked ash (Sarna- tion is andesitic. We have analyzed individual glass shards from both the Wojcicki and others, 1985). Only the lowest layer at Tulelake is likely to andesitic ash in the Tulelake core (sample T2023) and the andesitic ash- be primary airfall, the higher layers having been reworked from within the flow tuff (194M) in order to define the spread in chemical composition. basin. In Tulelake core 5, the Rockland ash bed is found at a depth of Both samples are polymodal, have a similar total compositional range, and 59.99 m (sample T1245), where it consists of an 11-cm-thick bed. have two identical modes intermediate in iron content (Table A). The Rockland ash bed has been found at numerous sites in northern The age of the andesitic tuff at Medicine Lake Volcano is constrained California and western Nevada (Sarna-Wojcicki and others, 1985; Table by K-Ar ages on older silicic domes (about 200 ka) that are overlapped by 2). Its source was the Lassen Peak area of northeastern California, al- the tuff, and younger andesite flows (about 100 ka) that overlie the tuff though the Rockland ash predates formation of Lassen Peak. The age of (J. M. Donnelly-Nolan and L. B. Pickthorn, 1989, written commun.). the Rockland ash bed, based on fission-track analyses on zircons, is 0.40 Donnelly-Nolan and Nolan (1986) have shown that the andesitic tuff at Ma (Meyer and others, 1991). This age is compatible with isotopic ages of Medicine Lake Volcano was probably erupted when ice covered the vol- the underlying Lava Creek-B ash bed (0.62 Ma), the overlying Loleta ash cano. In the Tulelake core, the ash bed of sample T2023 is situated only bed (= the Bend pumice; 0.3-0.4 Ma). about 6 m above the Rockland ash bed (about 410 ka; see below) and 33 m below the Blake Reversed-Polarity Subchron and its closely associated Tephra Layer of Sample T2119L; Ash 7, Clear Lake, California tephra layers (see above). Donnelly-Nolan and Nolan's evidence for an ice-covered Medicine Lake Volcano, combined with the age constraints at Within the same depth interval as the Rockland ash, there are tephra Medicine Lake, and our data from the Tulelake core, indicate that the layers of a different chemical composition. They are coarser and thus andesitic tuff was erupted during stage 6, or between about 136 ka and presumably from a closer source. Homogeneous tephra layers of this 180 ka (Imbrie and others, 1984). We choose the approximate mid-point chemical composition are found between 59.29 and 59.75 m in Tulelake of this interval, 160 ka, as the approximate age of the ash bed of sample core 2, with an individual ash bed at 59.35 m of depth (sample T2119,6 T2023; and we estimate an error of about 25 ka, approximately half the cm thick; sample T2119L, a pumice lapillus), and pumice lapilli at 59.75 time span of stage 6. m of depth (samples T2120L1 and T2120L2; Table A). A chemically Our previous age estimate of about 370 ka for the ash bed of sample similar, but more heterogeneous, tephra layer is present at 53.67 m (sam- T2023, used by Liddicoat and Bailey (1989), was based on straight-line ple T2038,5 cm thick; Table A). Another chemically similar tephra layer interpolation between the overlying Blake Reversed-Polarity Subchron is found at 61.55 m in Tulelake core 2 (spot sample T310), and at 61.57 m and the underlying Rockland ash bed. Evidence presented here indicates in core 5 (sample T2067, 4 cm thick). In the latter two samples, we that the previous estimate was in error. The younger age estimate of ash detected no MgO in the glass, whereas the seven samples from the higher bed T2023 also indicates that deposition in Tulelake was very rapid during tephra layers of this chemical type (depths between 53.67 and 59.75 m) the period from about 180 to 130 ka (Figs. 7a, 7b). contained concentrations of MgO that range from 0.01% to 0.09%. The tephra layer of Tulelake sample T2023 also correlates with A tephra layer in core 80-1 from Clear Lake, California (sample Summer Lake tephra bed KK at Summer Lake, Oregon (Davis, 1985) CL-0590), correlates well with the pumice lapillus at 59.35 m in Tulelake (sample DR-33; S.C. = 0.98). This correlation, and that of the ash bed of core 2 (sample T2119L; S.C. = 0.99). At Tulelake, the lowermost occur- sample T1193 with Summer Lake tephra bed V (Table 2), has made it rence of this chemical type of tephra is about 9 cm above the lowermost possible to estimate the ages of beds at Summer Lake that are bracketed by occurrence of the Rockland ash bed; consequently we consider this tephra these layers. The latter beds, in turn, have been correlated to tephra beds of to be younger than the Rockland ash bed. Because this compositional type upper Pleistocene lake sediments at Paoha Island, in Mono Lake, and at of tephra has been found outside the Tulelake basin only at Clear Lake, we Benton Crossing, near Lake Crawley, in east-central California, where the suggest a local volcanic source, possibly Medicine Lake Volcano. The layers are situated stratigraphically close to a magnetic excursion (Liddi- stratigraphic range of this tephra type at Tulelake implies that these chemi-

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cal types of tephra were produced over the period from about 400 ka to potential widespread temporal markers. Age estimates for these tephra 360 ka. layers based on the Tulelake sedimentation rate curve are given in Table 3. Analyses are found in Table A. Dibekulewe Bed

The Dibekulewe Bed of Davis (1978) is found in Tulelake core 2 at TABLE 3. SILICIC TEPHRA LAYERS BETWEEN 65.5 m AND 130 m DEPTH 62.07 m (spot sample T315), and at 62.27 m (spot sample T318); it is Depth Core Sample number Thickness Estimated age found in Tulelake core 5 at 62.02 m (sample T2069,1 cm thick), where it (m) number (cm) (Ma) overlies a tephra layer of slightly different composition at 62.10 m (sample 70.59 5 T1322 5 cm 0.67 T1249; 8 cm thick; Table A). Variation in chemistry and homogeneity 89.63 2 T432 I cm 0.80 between layers within this depth interval suggests that two or more chemi- 97.72 2 T452 3 cm 0.87 97.86 2 T2163 spot sample 0.87 cal types of silicic tephra are present. Although the Dibelukewe Bed is 99.52 2 T453 spot sample 0.88 101.94 2 T466 spot sample 0.90 widespread in northern California, western Nevada, and south-central 109.95 2 T2387 1 cm 1.04 Oregon, its source is unknown. The composition of its glass is intermediate 110.53 2 T2388 9 cm 1.06 125.69 2 T542 1 cm 1.35 between those of Long Valley caldera and the southern Cascade Range. The Dibekulewe Bed has not been dated but is found at several sites in stratigraphic context with dated tephra layers (Negrini and others, 1987) (Table 2); its age is between 0.41 and 0.62 Ma. Based on its position in Tulelake cores, we estimate its age at about 0.61 Ma and suggest that both Rio DeU(?) Ash Bed the Lava Creek B and the Dibekulewe eruptions occurred near the end of oxygen-isotope stage 16 (Sarna-Wojcicki and others, 1987; Sarna- Spot sample T558 from a tephra layer at 130.36 m in Tulelake core 2 Wojcicki and others, 1991). Note, however, that this estimate is subject to correlates reasonably well on the basis of electron-probe analysis (S.C. = error because the stratigraphic interval between the Rockland ash bed and 0.96) with the (informal) Rio Dell ash bed of Sarna-Wojcicki and others the Lava Creek-B ash bed at Tulelake (Figs. 6, 7a, 7b) is very short, (1982) (of the Rio Dell Formation of Ogle, 1953). The Rio Dell ash bed is suggesting a period of slow deposition or erosion during this time. The found in marine beds of the Humboldt Basin in northwestern California Dibekulewe Bed may lie at the base of, within, or above specific intervals (sample RD-2), and in deep-ocean cores from DSDP Holes 34 and 173 of slower sedimentation or erosion. (Tables A and 2; Sarna-Wojcicki and others, 1987). Its age, determined from a combination of magnetostratigraphic, biostratigraphic, and fission- Lava Creek-B Ash Bed track age data (Izett, 1981; Sarna-Wojcicki and others, 1987), is about 1.45 Ma. XRF analyses of bulk samples of glass separated from these The Lava Creek-B ash bed is identified in Tulelake core 2 at a depth tephra layers also indicate that Tulelake sample T558 correlates with the of 64.59 m (spot sample T329; Table A). The Lava Creek-B ash bed, Rio Dell ash bed (sample RD-2), although the titanium concentrations are erupted from the Yellowstone National Park area of northwestern Wyo- somewhat different. Such differences are typical of the chemically hetero- ming and eastern Idaho, is widespread across the central and western geneous Rio Dell ash bed. Tephra layer T558 at Tulelake lies above the United States (Izett, 1981; Izett and Wilcox, 1982; Table 2), and is found Olduvai Normal-Polarity Subchronozone (1.67 Ma) but below the Jara- in the eastern Pacific Ocean (Sarna-Wojcicki and others, 1984, 1987). Its millo Normal-Polarity Subchronozone (970 ka); the same stratigraphic age is 0.62 Ma, based on K-Ar and fission-track analyses (Christiansen and relationships are observed for the Rio Dell ash bed at the Centerville Blank, 1972; Naeser and others, 1973). Beach section in the Humboldt Basin.

Rye Patch Dam Bed Dormant Interval Represented in Tulelake Cores from 130.5 to 174 m in Depth The Rye Patch Dam Bed of Davis (1978), previously identified at Rye Patch Dam near Oreana in west-central Nevada and at Lone Mountain, The interval from about 130.5 m to about 174 m in Tulelake core 2 Eureka County, Nevada (Davis, 1987), is found in Tulelake core 2 at a contains few tephra layers; it represents a period of little volcanic activity depth of 65.38 m (spot sample T321; Tables A and 2). We also tentatively between about 1.5 and 1.8 Ma. One tephra layer at 166.41 m (spot sample identify the proximal correlative of this ash bed as the (informal) Desert T682) does not match well with any tephra layer of this approximate age Spring ash-flow tuff (Hill, 1985), exposed near Bend, Oregon. The Rye analyzed from outside the Tulelake basin. Patch Dam Bed is undated but is of normal magnetic polarity (J. O. Davis, 1984, personal commun.). From the Tulelake sedimentation-rate curve Tuffaceous Interval between Depths of about 174 to 181 m; (Fig. 7b), we estimate its age at about 0.63 Ma. Bear Gulch Ash Bed

Dormant Interval Represented in Tulelake Cores from 65.5 Within the depth interval 173.8 to 181.2 m, Tulelake core 2 contains to 130 m in Depth multiple, thick, coarse-grained tephra layers that were probably erupted from nearby vents (Table A; Fig. 7a). Interpolation between the base of The depth interval from 65.5 m to 130 m in the Tulelake cores the Olduvai Normal-Polarity Subchronozone and the top of the contains few tephra layers and represents a relatively dormant period of Reunion(?) Normal-Polarity Subchronozone gives ages between 1.92 and volcanic activity. Several silicic ash beds within this interval, however, are 1.93 Ma. The lowest tephra layer of this sequence, at 181.2 m (sample

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T726), is about 4 m below the main sequence; we estimate the age of this DISCUSSION ash bed to be about 1.94 Ma. These chemically similar, but heterogeneous, tephra layers represent The sedimentation-rate curves of Figures 7a and 7b are based on the coarse, plinian, proximal airfall tephra and (presumably) elutriated fine identified magnetostratigraphic horizons and tephrochronologic horizons ash from vents that produced the chemically similar welded and unwelded dated independently outside the Tulelake basin. The accuracy of our age ash flows exposed immediately to the west of Tulelake and south of Lower estimates for undated or poorly constrained horizons, derived from these Klamath Lake (J. M. Donnelly-Nolan, 1985, personal commun.). Both curves, depends on accuracy of the independent dates and the stratigraphic the airfall tephra and ash-flows came from an unknown source in the distance from the dated horizons. We assumed uniform rates of sedimenta- general vicinity of Tulelake, but they predate the formation of Medicine tion between adjacent age-control points, but we acknowledge that this Lake Volcano (J. Donnelly-Nolan, 1985, personal commun.). assumption is obviously not entirely valid because of unrecognized or The (informal) Bear Gulch ash bed (Sarna-Wojcicki and others, unconstrained variations in accumulation rate; we have noted instances 1982; sample SM-ASH-07) is found stratigraphically below the Rio Dell where significant, but unknown, variations appear likely. Thus, age esti- ash bed in the Rio Dell Formation (Ogle, 1953) in the Humboldt basin, mates cited here for stratigraphic datums other than the primary, inde- northeastern California (Tables A and 2). It is chemically similar to the pendently dated, age-control points should be considered approximate and tephra layers within the interval 174 to 181 m in the Tulelake cores. used accordingly. Nonetheless, the age estimates and stratigraphic relations Although the chemical match between Tulelake sample T715 and the from this single, thick section add considerably to the chronostratigraphic Bear Gulch ash bed is not as good as it is for some of the other correlations framework of a wide region of the western United States and adjacent presented here (S.C. = 97), the correlation is likely, because the strati- Pacific Ocean. Maximum errors in our age estimates in the uppermost 23 graphic positions and estimated ages of these tephra layers are compatible. m of the section, where independently dated horizons are relatively Differences between the proximal tephra and the inferred distal correlative abundant, are small—probably less than a few thousand years. Maximum layer may be a result of rapid multiple eruptions of this tephra sequence at errors over the depth interval 23 m to 57 m may be on the order of 20 ka Tulelake, the heterogeneous chemical composition of the proximal layers, to 40 ka, particularly if climatic and other factors affected short-term or the elutriation of fines from multiple ash flows. variability in sedimentation rates. Lower in the cores, errors in our esti- mates may be larger for horizons away from primary age-control points. Dormant Interval between 181 and 331 m in Depth Despite fluctuations in accumulation rate, deposition in the Tulelake basin was reasonably continuous during the past 3 m.y., averaging about 1 Below 181m depth, the Tulelake section contains few tephra layers m/104 yr. Figure 7b, representing the past 750,000 yr, is sufficiently and thus records a long period of dormancy from about 1.95 to about 3.01 detailed that fluctuations on the order of 104 to 105 yr are revealed. Some Ma. Several thin tephra layers present within this interval, however, may of these fluctuations, such as those during the past 125 ka, seem to corre- become important temporal horizons for correlation. No correlation with late at least in part with fluctuations in marine oxygen-isotope ratios dated or undated tephra layers outside of the Tulelake area have been (Shackleton and Opdyke, 1973; Imbrie and others, 1984) and may have made for these layers. The depths of these tephra layers in Tulelake core 2 been controlled by past changes in climate. Variations in apparent accumu- are given in Table 4; we made approximate age estimates for them using lation rate over some other intervals do not seem to correspond to marine the accumulation rate curve (Fig. 7a). Analyses are found in Table A. oxygen-isotope stages and may have resulted from factors other than climate. Two significant departures from continuous sedimentation are ob- TABLE 4. TEPHRA LAYERS BETWEEN 181 m AND 331 m DEPTH served in the Tulelake record. One, between about 52 and 63 m in depth,

Depth Core Sample Thickness Estimated age occurred between about 610 ka and 170 ka (Figs. 7a, 7b). Within this time (m) number (cm) (Ma) interval, slow or sporadic deposition, nondeposition, and possibly erosion

191.35 2 T1894 5 cm 1.97 occurred. The period coincides with several long interstadial (interglacial) 204.41 2 T782 spot sample (4 cm) 2.01 stages equivalent to marine oxygen-isotope stages 6b, 7,9,11,13, and 15; 206.04 2 T786 spot sample 2.02 213.42 2 T802 spot sample 2.05 and comparatively short stadial (glacial) stages, equivalent to stages 6c, 8, 225.93 2 T834 spot sample (1 cm) 2.31 265.21 2 T1417 5 cm 2.67 10, 12, and 14 (Fig. 7b; Shackleton and Opdyke, 1973). The summed 268.20 2 T921 7 cm 2.69 duration of interstadials during this period is considerably longer than that 314.96 2 T1893 4 cm 2.95 of stadials by a ratio approaching three to one (320 k.y. to 120 k.y.; Fig. 7b). This ratio based on recalculation of stage boundary ages in core V28-238 (Shackleton and Opdyke, 1973, 1976) by assuming a constant The two tephra layers at depths of 204.41 and 213.42 m are chemi- sedimentation rate in that core and calibration points of 125 ka for the cally similar to tephra layers stratigraphically above the Nomlaki Tuff stage 5/6 boundary and 750 ka for the Brunhes/Matuyama Chron Member of the Tehama and Tuscan Formations (3.4 Ma) near Millville boundary. The summed duration of interstadials using the "smoothed and Inwood, east of the Sacramento Valley in northeastern California. The stacked" curve of Imbrie and others (1984)—a more generally applicable deepest tephra layer recovered from the Tulelake cores, at 314.98 m, curve and one presumably not subject to variations in sedimentation matches well in chemistry with a tephra layer found in a surface exposure rate—provides a similar result (325 k.y. to 150 k.y.). These observations at the south end of Klamath Lake Reservoir, at Chalk Bank Landing, west suggest that hotter/drier climates may have prevailed at Tulelake during of Tulelake. Preliminary magnetostratigraphic work, however, indicates this period, and may explain why accumulation slowed. A second interval that the layers are of different magnetic polarity and thus cannot be the of slow accumulation is noted between about 220 m and 230 m of depth, same (H. J. Rieck, unpub. data). corresponding to the time interval from 2.5 Ma to 2.1 Ma (Fig. 7a). A

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third, less pronounced, interval of slow deposition is observed between elsewhere in the core. It presumably represents a unique event within the about 17-m and 10-m depth, and corresponds to the time interval between 3-m.y. history of this basin. Because stage 6a appears to be one of the about 125 ka and 35 ka (Fig. 7b). This interval is shorter than the other longest stadials within the past 750 ka, and because it was preceded by a two, and includes oxygen-isotope stage 5, another hot/dry period between sequence of comparatively long interstadials, we infer that products of about 125 and 85 ka. weathering and the accumulation of alluvium, colluvium, and eolian mate- The extremely rapid rate of sedimentation inferred for the interval rial during this long, relatively dry period would be readily mobilized and from 52 to 17 m in the Tulelake core, corresponding in time to oxygen- transported into the basin during a long cool/wet period. Also contribut- isotope stage 6a (170 ka to 125 ka; Figs. 7a, 7b), is not approached ing to the rapid rate of deposition during stage 6a was increased local volcanism between about 200 and 100 ka, as indicated by the large number of tephra layers deposited in the Tulelake basin during this time TEPHRA LAYERS (Fig. 8). An alternative to climatically controlled fluctuations in sedimentation PER 100 ka rate at Tulelake is filling of the basin to capacity before the three periods of reduced accumulation rate, followed by accelerated accumulation only 0 8' ' 1»2 ' after the capacity of the basin was enlarged by tectonic activity or dam- ming by volcanic activity. At the present time, we do not know which of these several factors, or combination of factors, controlled sedimentation 0.2 rates in Tulelake. If sedimentation histories similar to that of Tulelake can be demonstrated for other lake basins in the western United States, local factors such as volcanism, tectonism, and basin capacity could be recog- 0.4 nized and regional climatic significance evaluated.

SUMMARY AND CONCLUSIONS 0.6 Mutually complementary magnetostratigraphic and tephrostrati- 0.8 graphic studies have resulted in a chronostratigraphic framework derived from the upper 331 m of late Pliocene-Holocene sediments at Tulelake, California. This framework bears on interpretation and correlation of 1.0 Figure 8. Minimum number deposits over much of the northwestern United States and adjacent Pacific of tephra layers in lake beds of Ocean basin. The chronology reveals a fairly continuous 3-m.y. record of Tulelake through the past 3 m.y. sedimentation in the Tulelake basin that records widespread volcanic and 1.2 are represented on the left by global magnetic events. It provides the largest, most complete data set from short horizontal lines adjacent to a single site for the study of the volcanic activity of this region, and this aids in further documenting and refining age control for less well known geo- 1.4 the time scale. Longer horizon- tal lines represent multiple teph- magnetic events of short duration during Quaternary time. ra layers of similar age, the At least 18 datum levels of well-known or closely bracketed age are 1.6 number indicated to the right. identified across the cored section. Deposition at Tulelake continued into Right side of figure shows the historic time, providing an upper bounding age. Seven well-dated paleo- minimum frequency of tephra magnetic reversals younger than 3.0 Ma are clearly recorded in the strata; 1.8 eruptions expressed as the num- their stratigraphic positions are constrained within a meter or less. Seven ber of tephra layers within each widespread tephra layers dated from outside the Tulelake basin and rang- 100-ka interval. ing in age from about 7 ka to about 1.45 Ma are identified. Ages of three 2.0 additional tephra layers are closely constrained by stratigraphic relations to numerical ages outside the Tulelake basin. 2.2 Sedimentation rate curves were derived from the independently dated datum levels that we have identified. These curves allow age estimates for additional magnetostratigraphic and tephrostratigraphic horizons, particu- 2.4 ] larly within the upper half of the section. Some of the tephrostratigraphic datum levels have been recognized only at Tulelake, but these may prove 2.6 to be widespread and thus valuable as chronostratigraphic markers. Six short stratigraphic intervals of stable, stratigraphically reproduci- ble, paleomagnetic inclinations significantly different from those expected 2.8 are here interpreted to record magnetic excursions and brief subchrons. Correlation of five of these short intervals with previously reported events is supported by the tephrochronologic data and other stratigraphic control. 3.0 ] Sixty-three tephra layers recognized as products of individual erup- tions have been analyzed. This number does not include all tephra layers

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