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Dallanave, E, Bachtadse V, Crouch EM, Tauxe L, Shepherd CL, Morgans HEG, Hollis CJ, Hines BR, Sugisaki S.  2016.  Constraining early to middle Eocene climate evolution of the southwest Pacific and Southern Ocean. Earth and Planetary Science Letters. 433:380-392.   10.1016/j.epsl.2015.11.010   AbstractWebsite

Studies of early Paleogene climate suffer from the scarcity of well-dated sedimentary records from the southern Pacific Ocean, the largest ocean basin during this time. We present a new magnetostratigraphic record from marine sediments that outcrop along the mid-Waipara River, South Island, New Zealand. Fully oriented samples for paleomagnetic analyses were collected along 45 m of stratigraphic section, which encompasses magnetic polarity Chrons from C23n to C21n (similar to 51.5-47 Ma). These results are integrated with foraminiferal, calcareous nannofossil, and dinoflagellate cyst (dinocyst) biostratigraphy from samples collected in three different expeditions along a total of similar to 80 m of section. Biostratigraphic data indicates relatively continuous sedimentation from the lower Waipawan to the upper Heretaungan New Zealand stages (i.e., lower Ypresian to lower Lutetian, 55.5 to 46 Ma). We provide the first magnetostratigraphically-calibrated age of 48.88 Ma for the base of the Heretaungan New Zealand stage (latest early Eocene). To improve the correlation of the climate record in this section with other Southern Ocean records, we reviewed the magnetostratigraphy of Ocean Drilling Program (ODP) Site 1172 (East Tasman Plateau) and Integrated Ocean Drilling Program (IODP) Site 131356 (Wilkes Land Margin, Antarctica). A paleomagnetic study of discrete samples could not confirm any reliable magnetic polarity reversals in the early Eocene at Site 1172. We use the robust magneto-biochronology of a succession of dinocyst bioevents that are common to mid-Waipara, Site 1172, and Site U1356 to assist correlation between the three records. A new integrated chronology offers new insights into the nature and completeness of the southern high-latitude climate histories derived from these sites. (C) 2015 Elsevier B.V. All rights reserved.

Dallanave, E, Muttoni G, Agnini C, Tauxe L, Rio D.  2012.  Is there a normal magnetic-polarity event during the Palaeocene-Eocene thermal maximum (similar to 55 Ma)? Insights from the palaeomagnetic record of the Belluno Basin (Italy) Geophysical Journal International. 191:517-529.   10.1111/j.1365-246X.2012.05627.x   Abstract

In the lowermost Eocene sedimentary record of Ocean Drilling Program Site 1262 (Leg 208, Walvis Ridge, South Atlantic Ocean), the presence of a similar to 53-kyr-long normal polarity event has been recorded within the similar to 2.55-Myr-long reverse polarity Chron C24r (similar to 53.355.9 Ma) and termed PalaeoceneEocene magnetic reversal (PEMR). The origin of the PEMR has been speculatively related to a change in the Earth's rotation rate that was in turn influenced by an abrupt overturning of the ocean-atmosphere circulation that occurred during the PalaeoceneEocene thermal maximum (PETM) at similar to 55 Ma. Such provocative genesis for a magnetic-polarity reversal demands the PEMR to be confirmed (or refuted) in additional PETM sections. Here, we present detailed palaeomagnetic and rock-magnetic data from the Forada and Cicogna sections of the Belluno Basin in NE Italy, which contain an expanded and continuous record of the PETM termed clay marl unit (CMU). Our data indicate that these sediments were deposited during a continuous interval of reverse geomagnetic field polarity. We therefore conclude that no magnetic-polarity reversals occurred throughout the PETM. In addition, we studied the origin of the high degree of flattening affecting the characteristic magnetic component directions of the sediments, which we interpret as due to a combination of depositional inclination shallowing typical of detrital haematite, and post-depositional compaction of clays, particularly abundant in the carbonate-depleted CMU.

Tauxe, L, Stickley CE, Sugisaki S, Bijl PK, Bohaty SM, Brinkhuis H, Escutia C, Flores JA, Houben AJP, Iwai M, Jimenez-Espejo F, McKay R, Passchier S, Pross J, Riesselman CR, Rohl U, Sangiorgi F, Welsh K, Klaus A, Fehr A, Bendle JAP, Dunbar R, Gonzalez J, Hayden T, Katsuki K, Olney MP, Pekar SF, Shrivastava PK, van de Flierdt T, Williams T, Yamane M.  2012.  Chronostratigraphic framework for the IODP Expedition 318 cores from the Wilkes Land Margin: Constraints for paleoceanographic reconstruction. Paleoceanography. 27   10.1029/2012pa002308   AbstractWebsite

The Integrated Ocean Drilling Program Expedition 318 to the Wilkes Land margin of Antarctica recovered a sedimentary succession ranging in age from lower Eocene to the Holocene. Excellent stratigraphic control is key to understanding the timing of paleoceanographic events through critical climate intervals. Drill sites recovered the lower and middle Eocene, nearly the entire Oligocene, the Miocene from about 17 Ma, the entire Pliocene and much of the Pleistocene. The paleomagnetic properties are generally suitable for magnetostratigraphic interpretation, with well-behaved demagnetization diagrams, uniform distribution of declinations, and a clear separation into two inclination modes. Although the sequences were discontinuously recovered with many gaps due to coring, and there are hiatuses from sedimentary and tectonic processes, the magnetostratigraphic patterns are in general readily interpretable. Our interpretations are integrated with the diatom, radiolarian, calcareous nannofossils and dinoflagellate cyst (dinocyst) biostratigraphy. The magnetostratigraphy significantly improves the resolution of the chronostratigraphy, particularly in intervals with poor biostratigraphic control. However, Southern Ocean records with reliable magnetostratigraphies are notably scarce, and the data reported here provide an opportunity for improved calibration of the biostratigraphic records. In particular, we provide a rare magnetostratigraphic calibration for dinocyst biostratigraphy in the Paleogene and a substantially improved diatom calibration for the Pliocene. This paper presents the stratigraphic framework for future paleoceanographic proxy records which are being developed for the Wilkes Land margin cores. It further provides tight constraints on the duration of regional hiatuses inferred from seismic surveys of the region.

Krijgsman, W, Tauxe L.  2004.  Shallow bias in Mediterranean paleomagnetic directions caused by inclination error. Earth and Planetary Science Letters. 222:685-695.   10.1016/j.epsl.2004.03.007   AbstractWebsite

A variety of paleomagnetic data from the Mediterranean region show a strong bias toward shallow inclinations. This pattern of shallow inclinations has been interpreted to be the result of (1) major northward terrane displacement, (2) large nondipole components in the Earth's magnetic field, and (3) systematic inclination flattening of the paleomagnetic directions. Here, we use the observation that, in addition to the well-known variation of magnetic inclination with latitude, the N-S elongation of directional dispersion also varies, being most elongate at the equator and nearly symmetric at the poles. Assuming that inclination shallowing follows the relationship long known from experiment, we invert the inclinations using a range of "flattening factors" to find the elongation/inclination pair consistent with a statistical model for the paleosecular variation. We apply the so-called "elongation/inclination" method to the extensive paleomagnetic data sets from the Miocene sediments of the Calatayud basin (Spain) and the island of Crete (Greece). After correction, the Spanish data are in good agreement with the expected middle Miocene latitude of the region. The data from Crete suggest that it occupied a position in the late Miocene about 275 km north of the predicted location. This is in agreement with the geological and geodynamical models for the east Mediterranean region, which indicate that slab rollback processes in combination with Anatolian push generated southward migration of Crete. The 7.5 million year average displacement rate of Crete estimated by the E/I method is 37 mm/yr to the south, which closely coincides with present-day rates based on global positioning system (GPS) and model measurements. We also show that inappropriate tilt corrections lead to a shallow inclination bias as well, explaining that observed in studies of lava flows of the region. We conclude that the east Mediterranean inclination anomaly is caused by sedimentary inclination error and not by a persistent octupolar contribution to the geomagnetic field, or northward transport. (C) 2004 Elsevier B.V. All rights reserved.

Bowles, J, Tauxe L, Gee J, McMillan D, Cande S.  2003.  Source of tiny wiggles in Chron C5: A comparison of sedimentary relative intensity and marine magnetic anomalies. Geochemistry Geophysics Geosystems. 4   10.1029/2002gc000489   AbstractWebsite

[1] In addition to the well-established pattern of polarity reversals, short-wavelength fluctuations are often present in both sea-surface data ("tiny wiggles'') and near-bottom anomaly data. While a high degree of correlation between different geographical regions suggests a geomagnetic origin for some of these wiggles, anomaly data alone cannot uniquely determine whether they represent short reversals or paleointensity variations. Independent evidence from another geomagnetic recording medium such as deep-sea sediments is required to determine the true nature of the tiny wiggles. We present such independent evidence in the form of sedimentary relative paleointensity from Chron C5. We make the first comparison between a sedimentary relative paleointensity record (ODP Site 887 at 54degreesN, 148degreesW) and deep-tow marine magnetic anomaly data (43degreesN, 131degreesW) [ Bowers et al., 2001] for Chron C5. The sediment cores are densely sampled at similar to2.5 kyr resolution. The inclination record shows no evidence for reverse intervals within the similar to1 myr-long normal Chron C5n.2n. Rock magnetic measurements suggest that the primary magnetic carrier is pseudo-single domain magnetite. We choose a partial anhysteretic magnetization (pARM) as our preferred normalizer, and the resulting relative paleointensity record is used as input to a forward model of crustal magnetization. We then compare the results of this model with the stacked deep-tow anomaly records. The two records show a significant degree of correlation, suggesting that the tiny wiggles in the marine magnetic anomalies are likely produced by paleointensity variations. An analysis of our sampling density suggests that if any reverse intervals exist at this site, they are likely to be <5 kyr in duration. Furthermore, we suggest that reverse intervals during Chron C5n.2n documented in other locations are unlikely to be global.

Shackleton, NJ, Hall MA, Raffi I, Tauxe L, Zachos J.  2000.  Astronomical calibration age for the Oligocene-Miocene boundary. Geology. 28:447-450.   10.1130/0091-7613(2000)28<447:acafto>;2   AbstractWebsite

The stratotype section for the base of the Miocene is at a reversed (below) to normal (above) magnetic transition that is claimed to represent magnetic chron C6Cn.2n (o). Deep Sea Drilling Project (DSDP) Site 522 is the only location we are aware of that unambiguously records the three normal events of C6Cn. We have quantitatively determined the range of the short-lived nannofossil Sphenolithus delphix and the tower limit of S. disbelemnos in DSDP Holes 522 and 522A in order to calibrate their precise relationship to the magnetostratigraphy and to confirm the completeness of the record at this site. Astronomical tuning of Ocean Drilling Program (ODP) Sites 926, 928, and 929 shows that S. disbelemnos appears at 22.67 Ma and that the entire range of S. delphix is from about 22.98 Ma to 23.24 Ma. Using these ages, linear interpolation in DSDP Site 522 suggests that the age of C6Cn.2n (o) and of the Oligocene-Miocene boundary is 22.92 +/- 0.04 Ma. Our value, conservatively expressed as 22.9 +/- 0.1 Ma, is 0.9 m.y. younger than the currently accepted age of the Oligocene-Miocene boundary and of C6Cn.2n (o), which was assigned an age of 23.8 Ma, based on an estimate of 23.8 +/- 1 Ma for the Oligocene-Miocene boundary. The bulk-sediment carbon isotope data from DSDP Site 522 is correlated to the record from benthic foraminifera at ODP Site 929 to refine the calibration of magnetic reversals from C6Cn.1n (o) to C7n.2n (o) at DSDP Site 522 on the astronomical time scale.

Hicks, JF, Obradovich JD, Tauxe L.  1999.  Magnetostratigraphy, isotopic age calibration and intercontinental correlation of the Red Bird section of the Pierre Shale, Niobrara County, Wyoming, USA. Cretaceous Research. 20:1-27.   10.1006/cres.1998.0133   AbstractWebsite

The Red Bird section of the Pierre Shale in eastern Wyoming contains a relatively complete sequence of fine-grained marine clastics that were deposited between 81 and 69 million years ago in the Late Cretaceous epicontinental seaway of the US Western Interior. These units not only contain a well-studied, high-resolution ammonite biostratigraphic sequence, by which the far-hung exposures of the seaway sediments are correlated across this region, but they are also isotopically well-dated due to the presence of numerous sanidine-bearing volcanic ash layers. The magnetostratigraphy of the Red Bird section consists of three geomagnetic reversals which can be independently calibrated by seven Ar-40/Ar-39 isotopic ages in an interval that spans 12 million years of the Campanian and Maastrichtian stages. The magnetostratigraphic section can be confidently correlated to that part of the geomagnetic polarity rime scale (GPTS) that ranges from the base of subchron C33n to the base of C31n. Linear interpolation and extrapolation from the isotopic ages gives the following age estimates for these reversal boundaries: C32n/C31r, 70.44 +/- 0.7 Ma; C31r/C31n, 69.01 +/- 0.5 Ma. The C33n/C32r reversal boundary cannot be identified with complete confidence but it is certainly younger than the 74.62 +/- 1.2 Ma age interpolated for the reversal found at the top of C33n. These age estimates make a significant contribution to the calibration of the GPTS for the Cretaceous Period, which has previously relied heavily on interpolation between three or fewer calibration points that are widely spaced in age. In addition, the recognition of the chrons C33 through C31 in this section enables us to correlate the high resolution ammonite zonation of the US Western Interior directly to the time-equivalent European pelagic microfossil zonation based on the magnetostratigraphic reference section at Gubbio in north-central Italy. (C) 1999 Academic Press.

Tauxe, L, Herbert T, Shackleton NJ, Kok YS.  1996.  Astronomical calibration of the Matuyama-Brunhes boundary: Consequences for magnetic remanence acquisition in marine carbonates and the Asian loess sequences. Earth and Planetary Science Letters. 140:133-146.   10.1016/0012-821x(96)00030-1   AbstractWebsite

We have compiled 19 records from marine carbonate cores in which the Matuyama-Brunhes boundary (MBB) has been reasonably well constrained within the astronomically forced stratigraphic framework using oxygen isotopes. By correlation of the delta(18)O data to a timescale based on astronomical forcing, we estimate astronomical ages for each of the MBB horizons. In all but one record the MBB occurs within Stage 19. Most magnetostratigraphic sections in Asian Loess place the MBB within a loess interval. Since loess deposition is presumed to be associated with glacial intervals, loess horizons should correspond to even-numbered oxygen isotope stages. A glacial age for the MBB is at odds with the results presented here, which firmly place the MBB within interglacial Stage 19. Inconsistency among the many loess sections and between the loess and the marine records suggests that the magnetic interpretation of loess sections may be more complicated than hitherto supposed. The mean of the Stage 19 age estimates for the MBB is 777.9 +/- 1.8 (N = 18). Inclusion of the single Stage 20 age results in a mean of 778.8 +/- 2.5 (N = 19). The astronomical age estimate of the MBB compares favorably with an (unweighted) mean of 778.2 +/- 3.5 (N = 10) from a compilation of Ar-40/Ar-39 results of transitional lava flows. Combining the two independent data sets yields a grand mean of 778.0 +/- 1.7 (N = 28). The new compilation shows virtually no trend in placement of the MBB within isotope Stage 19 as a function of sediment accumulation rate. We interpret this to mean that the average depth of remanence acquisition is within a few centimeters of the sediment-water interface. Separating the cores into two geographic regions (an Indo-Pacific-Caribbean [IPC] Group and an Atlantic Group) results in a significant difference in the position of the mid-point of the reversal with respect to the astronomical time scale. The data presented here suggest a difference of several thousand years between the two regions. This observation could be caused by systematic differences between the two regions in sedimentation rate within the interval of interest, systematic differences in remanence acquisition, or by genuine differences in the timing of the directional changes between the two regions.

Harris, JD, Johnson KR, Hicks J, Tauxe L.  1996.  Four-toed theropod footprints and a paleomagnetic age from the Whetstone Falls Member of the Harebell Formation (Upper cretaceous: Maastrichtian), northwestern Wyoming. Cretaceous Research. 17:381-401.   10.1006/cres.1996.0024   AbstractWebsite

The Harebell Formation is a syntectonic sequence of conglomeratic sediments deposited in a narrow, rapidly subsiding trough that formed in the latest Cretaceous along the eastern margin of the ancestral uplift of what are today the Teton and Gros Ventre Mountains of northwestern Wyoming. On at least two occasions subsidence temporarily exceeded the rate of sediment supply and the area was flooded by a brackish or marine incursion from the Western Interior Seaway that lay to the east. The age of the Harebell Formation is Maastrichtian, corroborated by K-40/Ar-40 isotropic ages, vertebrate and palynomorph biostratigraphy, and a preliminary magnetostratigraphic analysis which correlates it to the geomagnetic reversal time scale from the upper part of C31R to the base of C30N. Sandstone slabs collected from the lower Whetstone Falls Member contain nine partial and complete footprints attributable to a theropod (Dinosauria: Saurischia). The footprints were formed as surface tracks in the tabular-bedded sandstone by dinosaurs that roamed the burrowed and leaf-littered sand flats and shallow waters along the margins of a low-energy, brackish-water embayment. Eight of the nine footprints represent a hitherto unknown ichnogenus, representing a four-toed pedal morphology for a theropod dinosaur which is unprecedented in the Late Cretaceous. The theropod nature of the tracks is implied by the length and narrowness of the digits and the sharp claw impressions. The tracks have clearly defined impressions of four toes, none of which appears to be a hallux in the traditional theropod sense of a small, retroverted hallux. The metapodial impression is also unlike that of other known theropod tracks: greater in relief than the digits but quite small in area. The tracks represent at least two individuals, although no clear trackways are available. Exallopus lovei, gen. et sp. nov., represents a type of theropod not currently recognized from body fossils. (C) 1996 Academic Press Limited

Behrensmeyer, AK, Potts R, Plummer T, Tauxe L, Opdyke N, Jorstad T.  1995.  The Pleistocene Locality of Kanjera, Western Kenya - Stratigraphy, Chronology and Paleoenvironments. Journal of Human Evolution. 29:247-274.   10.1006/jhev.1995.1059   AbstractWebsite

Kanjera is well known as the source of controversial hominid fossils collected by L. S. B. Leakey in the 1930s. Since 1935, the context of fossils and artifacts from the locality has been in doubt, due to a claim that sediment slumping had commingled materials from stratigraphic units of different ages. A careful re-examination of the geology demonstrates that the Kanjera deposits consist of approximately 37 m of volcaniclastic, fluvial, mudflat and lacustrine sediments that we assign to three major units: the Kanjera Formation, the Apoko Formation, and the Black Cotton Soil, Outcrops cover approximately 2 km(2) in two adjacent areas, the Northern and Southern Exposures. Fossils and artifacts are found in primary contexts through much of the stratigraphic column; and extensive trenching failed to reveal any sediment slumping that would have disturbed these contexts. Faulting, rapid lateral facies changes, and an erosional unconformity between the Kanjera and Apoko Formations result in complex geological relationships. Magnetostratigraphic and faunal determinations indicate that the Kanjera Formation is approximately 1.5-0.5 Ma, the Apoko Formation younger than 0.5 Ma, and the Black Cotton Soil latest Pleistocene to Holocene. The hominid sample is derived from the Black Cotton Soil except for Leakey's Hominid 3, which probably was an intrusive burial into Kanjera Formation Bed KN-2. The Theropithecus oswaldi type specimen originated from KN-2a and is dated between 1.1 and at most 1.76 Ma. The Kanjera Formation provides the youngest known records of Metridiochoerus andrewsi and Deinotherium bozasi at about 1.0 Ma. (C) 1995 Academic Press Limited

Hicks, JF, Obradovich JD, Tauxe L.  1995.  A New Calibration Point for the Late Cretaceous Time-Scale - the Ar-40/Ar-39 Isotopic Age of the C33r/C33n Geomagnetic Reversal from the Judith River Formation (Upper Cretaceous), Elk Basin, Wyoming, USA. Journal of Geology. 103:243-256. AbstractWebsite

We present a new calibration point for the latest Cretaceous time scale, an interval that at present contains no well-dated polarity reversals. The magnetostratigraphy of Campanian-aged sediments in the northern Bighorn basin of Wyoming documents a geomagnetic reversal in the upper part of the Judith River Formation that occurs in direct association with ash fall layers dated by the Ar-40/Ar-39 method. Extrapolation based on three new Ar-40/Ar-39 laser fusion dates from sanidines extracted from the ash fall layers dates the reversal at 79.34 Ma; thus the geomagnetic reversal in the Judith River formation can be confidently correlated to the C33r/C33n boundary of the Geomagnetic Reversal Time Scale. This age estimate is in good agreement with recent time scales that date this polarity interval by interpolation between few and widely spaced calibration points. The Judith River Formation is nonmarine and cannot be correlated regionally by traditional ammonite biostratigraphic methods. This study shows that in northern Wyoming the formation was deposited from 80 to 79 Ma, and can be correlated both magnetostratigraphically and by isotopic age to the Campanian-aged ammonite zones of the marine record that range from the top of Baculites obtusus to the lower part of B. perplexus (early form). The Judith River is underlain by the Claggett Shale, which contains one of the ash fall levels of the Ardmore bentonite. This ash fall is an important datum level found throughout the Western Interior U.S. in the range zone of B. obtusus, one we have dated at 80.71 +/- 0.55 Ma.

Hartl, P, Tauxe L, Herbert T.  1995.  Earliest Oligocene Increase in South-Atlantic Productivity as Interpreted from Rock Magnetics at Deep-Sea Drilling Project Site-522. Paleoceanography. 10:311-325.   10.1029/94pa03150   AbstractWebsite

The magnetic properties of the sediments (''rock magnetics'') at DSDP Site 522 in the South Atlantic exhibit clear differences between the latest Eocene and earliest Oligocene. Based on low temperature behavior of saturation remanence and hysteresis loops, we attribute the difference to a slightly greater proportion of the finest grained, so-called ''superparamagnetic'' magnetite in the Eocene sediments. We believe that the lower proportion of very fine-grained magnetite in the Oligocene sediments is a result of incipient reduction diagenesis caused by increased productivity and hence increased labile organic carbon transport to the sediments due to an early Oligocene increase in thermohaline circulation. The Eocene-to-Oligocene transition at Site 522 is also expressed by changes in microfossil assemblages, increased carbonate content, decreased insoluble residue, and decreased foraminiferal shell fragmentation. The increase in carbonate is synchronous with and parallels a change in the ratio of two of the rock magnetic parameters, a ratio that tracks the decrease in the very fine-grained magnetite component. Also parallel to these is a trend toward heavier delta(13)C values in foraminiferal tests. The increase in organic carbon transport to the sediments led to chemical dissolution of the finest grain-size fraction of magnetite in the Oligocene sediments, hence a reduction in the superparamagnetic component and the change in the rock magnetic ratio. In this way, rock magnetics can be sensitive indicators of environmental changes, such as fluctuations in organic carbon transport, which may leave little other trace in the sedimentary record.

Gallet, Y, Gee J, Tauxe L, Tarduno JA.  1993.  Paleomagnetic analyses of short normal polarity magnetic anomalies in the Matuyama Chron. Proceedings of the Ocean Drilling Program, Scientific Results. 130:547-559.   10.2973/   Abstract

We document three short normal intervals in the natural remanent magnetization of sediments within the Matuyama Chron. These three anomalous zones of magnetization between the Jaramillo and Olduvai subchrons were identified from continuous measurements of archive halves from Hole 803 A using the pass-through 2G cryogenic magnetometer at Scripps. The U-channel samples were taken from the three intervals, analyzed using the pass-through system, and then cut into discrete 1 -cm-thick samples. Measurements on discrete samples confirmed the presence of the upper normal polarity zone. Based on sedimentation rate calculations, this zone is confidently correlated with the Cobb Mountain Subchron. For the two other anomalous zones, complete thermal demagnetization revealed a high-stability component (250°-575°C) of reversed polarity. The intensity of the low-stability normal polarity component, normalized by susceptibility, remains roughly constant throughout the entire interval sampled, whereas the intensity of the high-stability reversed component is much lower within the normal zone than outside. We interpret these two normal zones, then, as periods of low (reversed polarity) geomagnetic field intensity resulting in low magnetization of the sediments; the periods of these low magnetization reversed polarity zones are completely masked by the component acquired viscously in a normal polarity field.

Tauxe, L, Monaghan M, Drake R, Curtis G, Staudigel H.  1985.  Paleomagnetism of Miocene East African Rift sediments and the calibration of the geomagnetic reversal time scale. Journal of Geophysical Research. 90:4639-4646., Washington, DC, United States (USA): American Geophysical Union, Washington, DC   10.1029/JB090iB06p04639   AbstractWebsite

Paleomagnetic stratigraphy and K-Ar age determinations are reported for the type section of the middle Miocene Ngorora Formation, found in the Kenya rift valley. The magnetostratigraphy is well correlated to the geomagnetic reversal time scale (GRTS) and spans from the lower part of Chron C5 (9) to Chron C5AB-r (14). K-Ar dates were determined for euhedral sanidines, handpicked from seven tuff horizons within the Ngorora Formation and the underlying Turn phonolite flow. These dates can therefore be tied directly to the GRTS. The eight dates fall into three discrete groups averaging 12.5 + or - 0.22 Ma (mean and standard deviation of results from four tuffs), 11.6 + or - 0.06 Ma (mean and standard deviation from three tuffs), and 10.16 + or - 0.38 (average of three analyses from one tuff). We interpret the age groups as resulting from three successive eruptive episodes, the stratigraphic positions of which are well constrained. In spite of episodic supply at the eruptive source, sediment accumulation is continuous at the resolution of the GRTS. This suggests that accumulation is controlled by basin subsidence rather than sediment supply. Sanidine dates support an age for the older boundary of marine magnetic anomaly 5 of about 10 Ma, as opposed to 11.12 Ma, suggested by the most recent results from Icelandic basaltic lava flows.

Poore, RZ, Tauxe L, Percival SF, Labrecque JL, Wright R, Petersen NP, Smith CC, Tucker P, Hsu KJ.  1983.  Late Cretaceous Cenozoic Magnetostratigraphic and Biostratigraphic Correlations of the South-Atlantic Ocean - DSDP Leg-73. Palaeogeography Palaeoclimatology Palaeoecology. 42:127-&.   10.1016/0031-0182(83)90041-x   AbstractWebsite

DSDP Leg 73 sediment cores allow direct calibrations of magnetostratigraphy and biostratigraphy for much of the latest Cretaceous to Cenozoic in the mid-latitude South Atlantic Ocean. A complete record of the Cenozoic was not obtained, however, because strong dissolution, poor core recovery and intense core disturbance have masked the biostratigraphy or magnetostratigraphy over some intervals of all recovered sections. DSDP Leg 73 results are given.