Search

The Republican and early Imperial monuments of Rome are, for the most part, built of tuffs quarried from at least seven pyroclastic deposits erupted from nearby Monti Sabatini and Alban Hills volcanoes. Remarks by Vitruvius (2.7.1-5), field observations of the monuments, and petrographic and rock testing studies of samples from Roman quarries demonstrate that Roman builders developed a good knowledge of the diverse material properties of the tuffs over centuries of use and exposure. Measurements of compressive strength, specific gravity, water absorption and adsorption of water vapour confirm that the petrographic characteristics of each tuff lithology strongly influence its strength and durability. Early construction utilized weakly durable, soft or vitric tuffs such as Tufo del Palatine or Tufo Giallo della Via Tiberina that are susceptible to decay, as at Temple C (290 Bc) of the Largo Argentina Sacred Area. Late Republican structures. such as the Temple of Portunus (80-90 BC), employed somewhat durable, vitric-lithic Tufo Lionato reinforced with travertine, a durable limestone quarried near Tivoli. Roman builders selected the material properties of the tuffs to advantage for specific structural elements within large public monuments of the first century BC and the first century AD, as at the tabernae of the Forum of Caesar (46 BC), where an upper storey of lightweight Tufo Lionato is supported by robust, lithic-crystal Lapis Gabinus pillars and flat arches reinforced with travertine. The tuffs are not very durable building stones; Romans preserved them with protective stucco, and travertine and marble cladding. Their high water intake, coupled with direct exposure to rain, daily fluctuations in relative humidity and urban weathering at present makes them especially vulnerable to decay.

Authigenic K-feldspar occurs widely in the North American Midcontinent in an alteration profile developed on uppermost Precambrian rocks. In the study areas of west-central Wisconsin and southeast Missouri, the alterationprofiles are kaolinitic and interpreted as former saprolite formed by weathering prior to deposition of Upper Cambrian sandstone. K-feldspar in granitic alteration profiles of west-central Wisconsin occurs either as coarse-grained red crystals and veins replacing kaolinite or as overgrowths and fine-grained crystals in pore space of partly altered granite. In a granitic profile of southeast Missouri, K-feldspar replaces primary feldspar and probably kaolinite. In a diabasic profile, K-feldspar is disseminated and forms veins in saprolite, and it replaces the outer 3-5 cm of core-stones. Dating was conducted with the 4 0 Ar/ 3 9 Ar method using incremental heating. Observed spectral types are plateaus (6), near plateaus (3), undulatory (14), and stair-step (5). Undulatory patterns are attributed chiefly to recoil, and integrated ages of these are usually within the error of plateau ages where present, indicating a balanced enrichment/depletion of 3 9 Ar due to recoil. Stair-step spectra are attributed to severe recoil. Plateau 4 0 Ar/ 3 9 Ar ages of 446-427 Ma (Late Ordovician and Silurian) were obtained from coarse-grained K-feldspar replacing kaolinite in Wisconsin, suggesting a duration of the alteration episode of ∼19 m.y. However, potentially accurate nonplateau age measurements (integrated and near-plateau ages) suggest that alteration continued for a considerable time thereafter, possibly to ca. 399-395 Ma (Early Devonian). Integrated 4 0 Ar/ 3 9 Ar ages of ca. 362 Ma (Late Devonian and/or Early Mississippian) were obtained from fine-grained K-feldspar. Plateau ages of ca. 450 (Late Ordovician) were obtained from K-feldspar of the diabase profile in Missouri; duration of K-feldspar alteration in this region was at least ∼16 m.y. and may have been as long as ∼52 m.y. The Late Devonian-Early Mississippian K-feldspar formed at temperatures of ∼100 °C from saline fluid averaging ∼10% NaCl equivalent and having a δ 1 8 O value of ∼3.5%. This brine may have originated from the Michigan Basin, East-Central Iowa Basin, and/or the Illinois Basin, coincident with the Acadian orogeny. The Middle Ordovician to Early Devonian dates of K-feldspar do not generally correspond in time with orogenic activity at the continental margin, and the mechanism(s) for forming the K-feldspar are uncertain. Fluid sources proposed for some or all of the K-feldspar include the Michigan Basin, Taconic mountains, Reelfoot Rift, Illinois Basin, and Midcontinent epeiric seas. Middle Ordovician K-feldspar of Missouri very likely formed at elevated temperatures, but evidence of fluid temperature is lacking for other pre-Late Devonian K-feldspar.

Excavation in the previously little-explored western portion of Olduvai Gorge indicates that hominid land use of the eastern paleobasin extended at least episodically to the west. Finds included a dentally complete Homo maxilla (OH 65) with lower face, Oldowan stone artifacts, and butchery-marked bones dated to be between 1.84 and 1.79 million years old. The hominid shows strong affinities to the KNM ER 1470 cranium from Kenya ( Homo rudolfensis ), a morphotype previously unrecognized at Olduvai. ER 1470 and OH 65 can be accommodated in the H. habilis holotype, casting doubt on H. rudolfensis as a biologically valid taxon.

Lake Olduvai was a saline, highly alkaline closed-basin lake in which lacustrine sediments of Bed I and lower Bed II of Olduvai Gorge were deposited between ca. 1.92 and 1.70 Ma. The lake consisted of a lacustrine area, the central basin (CB), surrounded by lake-margin terrain that was alternately flooded and dried. The lacustrine deposits consist largely of authigenic clay and carbonate minerals, zeolites, K- feldspar, chert, and pyrite. Claystones of the CB and most of the eastern lake-margin (ELM) terrain consist chiefly of Mg-rich authigenic illite, smectite, and interlayered illite-smectite (I/S). These clays were formed penecontemporaneously with detrital-clay deposition by interaction of the detrital clays with the saline, highly alkaline water. The Lake Olduvai clays average ∼65%–70% Mg 2+ , SiO 2 , and alkali ions from lake water, and the remaining 30%–35% is detrital clay. Lacustrine clays of the ELM indicate that this area was flooded by saline water for substantial periods of time. Hominids occupied the ELM at times of lower lake levels, when the clays were dried. Calcite is the most abundant carbonate mineral and occurs chiefly as sand-size euhedral crystals dispersed in claystone and less commonly as calcite-crystal limestones and nodules. The sand-size calcite crystals were formed diagenetically at shallow depths, possibly from a gaylussite and/or micritic calcium carbonate precursor. Dolomite occurs chiefly in the form of dolostones, some of which are probably of replacement origin. The oxygen isotope composition of lake water and pore fluid of the CB varied over ∼11‰, as determined from calculated \({\delta}^{18}O_{H_{2}O}\) values of authigenic minerals. Clay minerals were formed chiefly in the more saline waters. Sand-size calcite crystals were generated mostly from less saline pore fluids, and limestone nodules were formed chiefly from dilute pore fluid. Dolostones were formed from fluids of varying salinity. Hydrogeology of the central basin was complex, and postdepositional fluid movements are documented by differing \({\delta}^{18}O_{H_{2}O}\) values for different authigenic minerals of the same samples. Pore fluids moved within the lake deposits for a considerable period of time after deposition, as shown by δ 18 O values of quartz and K-feldspar formed at different times. Downward flow of lacustrine fluids is indicated by zeolitic alteration of the underlying tuffaceous alluvial deposits. Lake Olduvai became overall shallower during the deposition of Bed I, and the subaerial exposure increased, leading to occasional playa conditions. These changes very likely resulted from increasing aridity and tectonic activity. The lake that deposited lower Bed II was larger than that responsible for upper Bed I, and episodes of wetter climate alternated with drier periods in which salinity and probably temperatures were higher than in the Bed I lake.

In the lower Paleozoic of the Illinois Basin, three illite polytypes are found: 2M 1 of detrital origin, and 1M d and 1M of diagenetic origin. Illite polytype quantification of detrital 2M 1 illite and diagenetic 1M d and 1M illite, combined with K-Ar age dating, allows extrapolation to apparent detrital and diagenetic illite ages. Kinetic modeling of smectite illitization, combined with the calculated age of illitization, can evaluate different origins of illite. The diagenetic illite in the lower Paleozoic of the Illinois Basin is interpreted not to have formed solely by burial diagenesis but mainly during multiple brine events. The Upper Ordovician Maquoketa Group contains diagenetic illite (dominantly 1M d with minor 1M ) with an extrapolated age of ∼360 m.y. (356–377 m.y.) and formed from smectite at temperatures of 50–100 °C. This age falls within the span of dates for illite/smectite (I/S) in K-bentonites from the Upper Mississippi Valley and is interpreted to be a combined result of illitization by burial diagenesis and either a hydrothermal brine from the southern and deeper part of the basin or a K-rich brine from the Michigan Basin, Upper Mississippi Valley area, or Forest City Basin. In Ordovician and Cambrian shale partings and sandstone older than the Maquoketa Group, the diagenetic illite ( 1M d in shale and 1M in sandstone) has an age of ∼300 m.y. and formed at temperatures

In contrast to those deposits formed by the alteration of volcanic ash in saline, alkaline lakes discussed in the chapter by Hay and Sheppard (this volume), large volumes of tuffaceous sediments around the world have been transformed to zeolites and other authigenic silicate minerals by the action of percolating water in open hydrologic systems. These include systems that have experienced significant chemical and hydrologic exchanges with the surrounding environment. Tephra sequences exposed to open-system alteration commonly show a vertical zonation of zeolites and other authigenic minerals that reflects the chemical changes in meteoric water moving through the system. Flow in an open hydrologic system can either be downward or have a downward component where meteoric water enters the system, resulting in a vertical or nearly vertical zonation of water composition and authigenic minerals. The original pyroclastic materials of tuffaceous sediments may have been laid down in the sea close to the volcanic sources, air-laid onto the land surface, or reworked into fluviatile and freshwater lacustrine environments. Open-system zeolite deposits are most common in nonmarine rocks, although many are known in sediments that were deposited in shallow marine environments. Zeolite deposits of the open-system type are commonly several hundred meters thick and can be traced laterally for several tens of kilometers. Many examples of zeolite deposits that formed from land-laid tephra have been recognized in the western United States, but most of the large marine deposits are in Japan and in southern and southeastern Europe. Because of their relatively large and commonly discontinuous areal extent, open-system deposits have not been studied in as much detail as some other types of zeolite deposits in sedimentary rocks; however, studies of several key areas have provided a sound basis for the current understanding of this type of zeolite body. Zeolitic alteration can take place …

Zeolites have been known since the mid-1750s, but prior to the early 1950s, most reported occurrences of zeolites were in fracture fillings and amygdules in igneous rocks, particularly basaltic lava flows. Indeed, most of the large attractive zeolite specimens in museum collections were obtained from lavas. In recent years, zeolites have been recognized as important rock-forming constituents in low-grade metamorphic rocks and in a variety of sedimentary rocks. Most zeolites in sedimentary rocks are finely crystalline, that is they occur as microscopic or submicroscopic crystals, and they are therefore of little appeal to mineral collectors; however, deposits of this type are voluminous and have great geologic significance and economic potential. Zeolites are among the most common authigenic silicate minerals that occur in sedimentary rocks, and they form in sedimentary rocks of diverse lithology, age, and depositional environment. About twenty species of zeolites have been reported from sedimentary rocks, but only eight zeolites commonly make up the major part of zeolitic rocks. These are analcime, chabazite, clinoptilolite, erionite, heulandite, laumontite, mordenite, and phillipsite. This chapter will consider chiefly the zeolites in sedimentary rocks, with emphasis on volcaniclastic deposits, which contain the largest concentrations of zeolites. The occurrence of zeolites in lava flows is mentioned only briefly. Journal articles on the occurrence and origin of natural zeolites have multiplied at a rapid rate since the Mineralogy and Geology of Natural Zeolites was first published in 1977 as Volume 4 of the Mineralogical Society of America’s Reviews in Mineralogy . The present review will highlight areas of more recent research on the occurrence and origin of zeolites and some of the coexisting minerals. A wide variety of zeolites has been identified in sedimentary deposits, with the most common being clinoptilolite, analcime, heulandite, laumontite, and phillipsite. Less abundant zeolites include chabazite, erionite, mordenite, natrolite …

Much controversy has arisen over both the depositional and diagenetic environments of the lacustrine sediments and the chemical evolution of waters within those sediments that make up the upper Oligocene Creede Formation. Previous studies of the neighboring Creede Ag-Pb-Zn district proposed that saline and isotopically heavy fluids from the Creede Formation were involved in ore deposition. Examination of rhythmically laminated limestones from two Continental Scientific Drilling Program drill cores (CCM-1 and CCM-2) provided a unique opportunity to study carbonate sediments of a caldera lake. The upper parts of CCM-2 (50–148 m depth) are characterized by laminations of low-Mg calcite containing brine shrimp fecal pellets (with an aragonite precursor), organic matter (including bacterial mat–like laminae), calcite pseudomorphs after gypsum, and cements of bladed low-Mg calcite and tabular clinoptilolite. The degree of recrystallization and crystal coarsening is greater in the lower part of CCM-2 (172–309.4 m depth) and in CCM-1 (8.8–155.2 m depth). These observations suggest that ancient Lake Creede was an evaporative, partially meromictic, saline to hypersaline, near-neutral, Na-Mg-SO 4 -Cl or Cl-SO 4 lake. SO 2− 4 and H + were supplied to the lake by the oxidation of volcanic SO 2(gas) , which buffered the alkalinity of the lake. Brine shrimp in the epilimnion are consistent with salinities greater than seawater and with warm surface temperatures. The lack of bioturbation in rhythmically laminated carbonate-siliciclastic couplets indicates either meromictic conditions or saline conditions or both. Bacterial mat-like laminae suggest that lake waters were either relatively shallow or clear. Displacively grown gypsum crystals indicate times when evaporative ground-water conditions dominated (e.g., during low lake levels), and their absence may indicate changes in water chemistry and lake level. Sulfate reduction during burial resulted in the conversion of gypsum and iron oxides to calcite and pyrite. Maximum estimates for the duration of the lake range from 132 323 to 76 340 yr and are based on rhythmites in CCM-2. Results from this study support the hypothesis of saline and isotopically heavy waters evolving within the Creede Formation prior to ore deposition.

This paper presents a 27Al and 29Si MAS NMR study of K-feldspars and demonstrates that the spectra are sensitive to variations in the state of Al,Si order. For synthetically annealed samples, the results are in agreement with previous IR spectroscopy (Harris et al., 1989) and demonstrate that Al,Si rearrangement continues after the samples have become monoclinic as determined by powder XRD. NMR methods provide a significantly improved picture of the state of local Al,Si order in such samples. For triclinic samples, measures of the state of Al,Si order (M1 and M2 of 27Al spectra and M2 of 29Si spectra) correlate well with site occupancies determined by powder XRD, but for the monoclinic samples the NMR parameters continue to change whereas the XRD parameters do not. Interpretations based on the NMR results for the synthetically disordered samples are consistent with 1-step disordering, as observed by XRD. 27Al and 29Si MAS NMR is likely to be a useful tool for probing the state of local Al,Si order in a wide variety of natural samples.

The Sinya Beds of the Amboseli Basin in Tanzania and Kenya consist largely of carbonate rocks and Mg-rich clays that are intensely deformed where exposed in and near former meerschaum mines. The carbonate rocks consist of limestone and dolomite in Tanzania, but only dolomite has been identified in Kenya. Sepiolite and mixed-layered kerolite/stevensite (Ke/St) are subordinate constituents of the carbonate rocks. The carbonate rocks and overlying bedded sepiolite were deposited in a semiarid lake basin at the foot of the large volcano Kilimanjaro. Calcite and dolomite of the carbonate rocks have ~tsO values 4-6%0 lower than calcite and dolomite of the late Pleistocene Amboseli Clays, suggesting that the Sinya Beds were deposited in the middle or early Pleistocene under a different climatic regime when meteoric water had lower 6~sO values than at present. Mg-rich clay minerals form veins and fill cavities in the Sinya Beds. The principal clay minerals are sepiolite and Ke/St, some of which contains substantial AI and Fe (A1-Ke/S0. NEWMOD | modeling and other X-ray diffraction (XRD) data suggest that most of the Ke/St contains 25-50 percent kerolite layers, but minor amounts ofkerolite-rich Ke/St are present in some samples. Illite with an inferred high content of Fe or Mg is a minor constituent of the samples with A1-Ke/St. The cavity-filling clays were chemically precipitated, as shown by field relationships and SEM study. The early-deposited clays of veins and cavities are principally Ke/St with minor sepiolite, and the latest clay is sepiolite (meerschaum), generally with minor Ke/St. The ~tsO values of cavity-filling Ke/St range from 22.5-25.6%0 and correlate with mineral composition, with the highest values associated with the highest content of stevensite and the lowest values with the highest content of kerolite. This relation suggests that high salinities favored stevensite and low salinities favored kerolite. ~sO values of sepiolite (meerschaum) fall in the middle of the range for Ke/St, suggesting that salinity was not the main control on sepiolite precipitation. High values of amo2/a~2§ may have been a major factor in sepiolite precipitation. Different mixtures of dilute ground water and saline, alkaline lake water in pore fluids may largely account for the differences in clay mineralogy of cavity-filling clays. Sepiolite is the dominant clay mineral in lacustrine sediments of the Amboseli Basin, and the cavity-filling sepiolite may reflect a high proportion of lake water. The low-A1 Ke/St may have formed from fluids with a higher proportion of ground water. Detrital clay was very likely a factor in forming the A1-Ke/St, for which ~mO values suggest a saline environment.

Smectite, opal-CT, and clinoptilolite are common alteration products in the White River sequence, a mid-Tertiary unit in the Great Plains and northern Rocky Mountains. The White River sequence is largely made up of fine-grained tuffaceous material that is rich in unaltered rhyolitic glass. Smectite is ubiquitous, and it or a precursor phase was present during White River soil formation but also formed after burial. Smectite Fe (from 5% to 7% Fe 2 O 3 ) and Ti (from 0.4% to 0.8% TiO 2 ) concentrations in mud rocks suggest formation from dacitic to latitic glass. Rhyotitic glass, however, was the primary source for opal-CT and clinoptilolite. Opal-CT and clinoptilolite have a highly variable distribution over macroscopic to regional scales of observation. Stable isotopic compositions of smectite (δ 18 O (SMOW) = 11.8‰ to 13.3‰), opal-CT (δ 18 O (SMOW) = 19.9‰ to 24.3‰), and late diagenetic calcite (δ 18 O (SMOW) = 10.3 ± 2.0‰) suggest that burial diagenetic reactions took place at temperatures ranging from 27 to 55 °C. Clinoptilolite apparently formed in areas with high contact times between paleoground water and vitroclastic material. In White River rocks of Wyoming, clinoptilolite occurs only in localities that are interpreted as lying within paleo-ground-water discharge zones. In the Great Plains, clinoptilolite is rare in eastern Colorado and southern Nebraska but progressively increases in abundance to the north, reflecting a northward decrease in permeability. Zeolitic alteration near Douglas, Wyoming, changed the hydrologic role of tuffs from aquifers to aquitards. Tuffs were originally aquifers because of their high Permeability (>30 md) relative to interbedded mud rocks (10 -4 to 1 md), Signigicant thicknesses (0.5 to 2 m presentday), and lateral continuity. Tuff alteration, however, decreased permeability by ∼4 orders of magnitude relative to glass-rich portions. Similar diagenetic effects on mud-rock permeabilities are likely. Mass flux into altered tuffs at Douglas is indicated by an increase in bulk rock density from ∼1.3 g/cm 3 for glass-rich tuff to ∼1.7 and ∼2.0 g/cm 3 , respectively, for zeolitic and siliceous counterparts. Local silica redistribution accouts for the bulk of the density increases. A systematic increase in Ba content of clinoptilolite over a 150-m vertical section (from 0.1 to 6.6 mol% of the exchangeable cations) suggests that it was derived from underlying units.

New geochronological data indicate that hominid-bearing deposits of middle to upper Bed I at Olduvai Gorge comprise an extremely brief interval of time, from about 1.80 to 1.75 Ma. Dates for lower Bed I suggest that the age of the lower boundary of the Olduvai subchron is >1.98 Ma, over 100 ka older than the currently accepted value. The results of this study enable rates of biological, archaeological, geological, and climatic change during Bed I times to be determined with great precision. The new date for the base of the Olduvai subchron has important implications for calibrating the age of marine and continental deposits that rely on the Olduvai subchron for chronologic control.

Potassium-argon dating of Bed I in Olduvai Gorge was done or supervised by G.H. Curtis and J.F. Evernden in the Department of Geology and Geophysics at the University of California at Berkeley. This work proceeded in three phases, the first of which was the dating of a small number of tuff samples by Evernden and Curtis. The dates ranged from 1.57 to 1.89 Ma, averaging 1.73 Ma, for tuffs interstratified with hominid-bearing deposits. This dating was controversial because of the long span indicated for the Pleistocene and for the antiquity of early tool-making hominids. Koenigswald et al. (1962) obtained a date of 1.3 ± 0.1 Ma for a sample of basalt from below the dated tuffs of Bed I and argued that the dates from tuffs were in error. Evernden and Curtis then dated the basalts at ⩾ 4.4 Ma, which was revised to 1.92 Ma, supporting the earlier dates on tuffs. The stratigraphy of Bed I was worked out over the period of dating, and some dates that made no sense early in the stratigraphic work fell into place later. The second phase of dating was primarily intended to check the earlier date of ∼1.75 Ma for hominid remains and stone artifacts of Bed I. This work was based on a large number of tuff samples and yielded a wide scatter of ages. Contamination by older materials was recognized in some samples and inferred in others, and criteria were established for accepting and rejecting dates. Using these criteria, 15 dates were considered as valid from the total of 41 dates obtained from tuffs above the basaltic lavas. The mean age of the valid dates was 1.74 ± 0.05 Ma, confirming the antiquity of the early hominids. The 15 valid dates were, however, difficult to interpret in terms of the stratigraphy, and further selection was made among the valid dates to estimate a time span of 350,000 years for the interval between the basalts (1.92 ± 0.06 Ma) and the highest tuff dated (1.57 ± 0.05 Ma). The third phase involved dating samples from previously undated horizons and re-evaluating all work done previously on Bed I samples. The highest priority for new dating was Tuff IF, at the top of Bed I, but the results were unsatisfactory, apparently because of undetected contamination by older volcanic materials. Remeasurement of the earlier mass spectograms led to changes in some of the published dates, and the re-evaluated dates, except for those of Tuff IB, still showed a wide scatter, indicating that most of the sampled tuffs were unsuited for accurate KAr dating. Only Tuff IB, with a mean age of 1.786 ± 0.029 Ma, gave results with a precision comparable to laboratory standards. The re-evaluated Bed I dates showed no trend toward younger ages upward, suggesting a relatively short time span for Bed I. Recent 40 Ar 39 Ar dates based on analyses of single crystals mark a great improvement over the KAr dating of Bed I. Dates on all of the marker tuffs fall in the correct stratigraphic order, and the age span of Bed I is ∼0.27 Ma, from 2.02 to 1.75 Ma. Components yielding two or more ages were identified in several tuffs, thus accounting for some and perhaps most of the variability in the KAr dates.

BED I of Olduvai Gorge has yielded some of the most important hominid fossils in the world, but precise age estimates have been elusive. Here we report new dating results for Bed I, based on 40Ar/39Ar analyses of single mineral grains using the laser-fusion technique. These dates reveal that the fossiliferous deposits of middle to upper Bed I, a period of significant biological and climatic change, comprise an extremely brief interval of time, from 1.80 Myr to 1.75 Myr. Dates for lower Bed I suggest that the base of the Olduvai subchron, of the geomagnetic polarity timescale, may be around 100,000 yr older than the currently accepted value.

Core KM-3 at Searles Lake, California, comprises 693.4 m of lacustrine sediments deposited over the past 3.2 m.y. The lake water evolved from moderately saline, slightly alkaline, and dominated by Na, Ca, C1, SO4, and HCO3 + CO3 to a highly alkaline brine dominated by Na and CO3 ions. Sediments are chiefly muds and evaporites. Montmorillonite and illite are the principal detrital clay minerals supplied to Searles Lake at present and probably during the late Pliocene and Pleistocene. The drill core is divided into three diagenetic zones on the basis of clay-mineral reactions. The upper zone (0-291.1 m) contains authigenic Fe-illite, Mg-smectite, K-feldspar, and analcime, which average 60-70% of the

The mineralogy, chemistry, and mineral textures of tuffs and tuffaceous sediments from two drill cores, CCM-1 and CCM-2, were studied to better understand depositional and diagenetic conditions of the Creede Formation of Colorado. Similarities between phenocrystic crystal assemblages and relict shard shapes and sizes in correlated tuffs indicate minimal offset between CCM-1 and CCM-2. Vertically zoned sequences of authigenic silicates in tuffs reflect alteration of silicic-intermediate vitric tuffs within an elevated geothermal gradient (100-110 °C/km). The abundance of clinoptilolite and Al-smectite in drill core CCM-2 is consistent with diagenetic waters having low to moderate salinity and a pH of 7-9. The higher-grade mineral assemblage, including analcime, quartz, minor heulandite, and albite, in CCM-1 reflects hotter fluids migrating through more permeable strata. The presence of kaolinite and mixed-layer clays in nontuff lithologies in CCM-1 indicates intermittently low cation/H+ ratios and SiO 2 ( a q ) activities. Alteration occurred in a relatively open chemical system based on calculated gains and losses from silicic to intermediate end-member compositions of Fisher Quartz Latite compared to Creede Formation tuffs. On the basis of comparisons with studies of zeolite formation in other burial diagenetic systems, the observed authigenic silicate assemblage (opal-CT, quartz, clinoptilolite, and analcime) in tuffs of the Creede Formation probably formed within ∼3-4 m.y. of deposition. This assemblage was later overprinted by a hydrothermal event (K-feldspar, I/S, C/S, chlorite, and quartz) in CCM-1 at 17.5 Ma. Authigenic silicate assemblages in tuffs poorly constrain the salinity of diagenetic pore waters because they could form in waters ranging from fresh to moderately saline with a pH ≤ 9. The inferred low to moderate salinities for silicate alteration and relatively open chemical system suggest that salinities were progressively lowered from moderate to high during deposition of ancient Lake Creede to lower salinities during silicate reactions associated with later burial diagenesis and hydrothermal alteration.

Natrocarbonatite and nephelinite tephra have been erupted together from Oldoinyo Lengai over the past few thousand years. The oldest deposits with recognizable natrocarbonatite are thin tuff beds probably between 2000 and 5000 years in age. Tuffs and agglomerates with an age of about 1250–2000 years contain evidence strongly suggestive of natrocarbonatite ash. The volcano is mantled by nephelinite-carbonatite ash deposits, termed the Footprint Tuff, that were erupted about 600 years ago. Evidence is contradictory as to whether natrocarbonatite was discharged during the cone-building phase of eruptions, which ended about 15,000 years ago. Nephelinite-carbonatite ash erupted in 1966 contains an unidentified mineral, designated NCS, with a chemical composition of Na4.09Ca2.76Si5O14.81. It may be genetically related to natrocarbonatite magma as it has thus far been identified only in the younger tephra deposits of Oldoinyo Lengai, most of which contain evidence of natrocarbonatite. A tephra deposit termed the Footprint Tuff contains footprints thought to be preserved by the rapid recrystallization of primary natrocarbonatite. Calcite of natrocarbonatite origin forms an estimated 15–20% of the airfall tuffs, and natrocarbonatite probably equalled or exceeded the volume of nephelinite tephra at the time of eruption. Nyerereite ([Na0.82K0.18]2 Ca[CO3]2) and gregoryite ([Na0.78K0.05] [Ca0.17CO3]) were primary minerals in the natrocarbonatite, as in modern lavas of Oldoinyo Lengai. Unlike modern lavas, the groundmass contained a substantial amount of silicate material. Noncarbonate minerals in the Footprint Tuff include nepheline, melilite, augite, wollastonite, melanite, fluorite, and NCS. The Footprint Tuff was cemented soon after deposition, very likely by trona. Gaylussite, pirssonite, or both, were probably later alteration products in the transformation of natrocarbonatite ash to form calcite. Oxygen and carbon isotopic re-equilibration were essentially decoupled in alteration of natrocarbonatite ash. δ18O values of this calcite suggest extensive to complete oxygen exchange, but δ13C values are in and near the range for unaltered natrocarbonatite. Carbon exchange may have been retarded in ash alteration by the high pH of pore fluid.

Recent excavation of the tuffs of the Laetolil Beds in Tanzania has revealed the presence of a large variety of footprints from the Pliocene. Many of these prints can be correlated with fossilised remains of Pliocene animals found in the same area.

A massive white sepiolite deposit at Amboseli precipitated from magnesium and silica released during the ground water dolomitization of an earlier lacustrine sepiolite. Kerolite has since formed in proximity to the massive sepiolite as an alteration product of sepiolite and as a ground water precipitate when the pH is below 8. Authigenic sepiolite also occurs in the overlying younger sediments. Kerolite is likely to occur but has not yet been positively identified. Presently, ground waters within the Amboseli Basin are supersaturated with respect to sepiolite and kerolite. This supersaturation results from the weathering of alkaline olivine basalts on the edge of the basin. The precipitation of sepiolite and/or kerolite does not control ground water compositions in the basin. These reactions are slow compared to other aqueous-mineral reactions such as those maintaining carbonate mineral equilibria. Equilibrium between disordered-dolomite and calcite buffers the log a Mg2+/(a H+)2 as a function of log f CO 2 in ground waters in the proximity of the massive sepiolite. This reaction can help explain the presence of sepiolite associated with dolomites in other near-surface deposits besides Amboseli.

This article reports the results of analysis of samples collected from some Egyptian quartzite artifacts. such as statuary, dating from the New Kingdom. Most of the artifacts are located in Upper Egypt, but the results of analysis suggest that, until the middle of the fourteenth century B.C., the quartzite came from a more distant quarry near Cairo rather than from the quartzite quarries located near Aswan in Upper Egypt.

Pleistocene man was surely aware of what were good materials for the fabrication of lithic artefacts, but no systematic studies have been conducted to test this proposition. The situation at Olduvai in Bed II offers an opportunity to extend the work of M. D. Leakey (1971) and R. L. Hay (I97I) and to examine many important questions: for example, the choice of raw materials by early hominids, the distances over which materials were carried, and the forms of flaked stone that were most valued and were therefore carried from factory sites to living sites. This paper reports on preliminary investigations to test certain hypotheses, directly or indirectly concerned with reconstructing hominid activity and behaviour in such matters. The term 'factory site' is here used to indicate a special activity site where full-time occupation, with all of the concomitant behaviour associated with a home base, is not in evidence. The density of the chert specimens, filling literally all of the space excavated, and the almost total absence of food debris, i.e. bones, in conjunction with large quantities of flaking debris, are the criteria on which the MNK Chert Factory Site (hereafter referred to as MNK CFS) is defined as a factory site. This paper describes the MNK CFS site and offers one example of a type of study developed to aid in explaining archaeological data in terms of human behaviour, in this case certain early hominid behaviour patterns in the exploitation of chert as a raw material for tool-making and utilization. Certain characteristics of the stone at MNK CFS are used as a basis for proposing working hypotheses to be tested by archaeological data furnished from elsewhere in the Olduvai Gorge. The method employed in this study is attribute analysis of a selected segment of the range of categories classified at MNK CFS and a subsequent comparison of this group with the corresponding artefacts from a site elsewhere in the gorge dating from approximately the same period, HWK East, levels

Carbonatite lava and tephra are now well known. The only modern eruptive carbonatites, from Oldoinyo Lengai, Tanzania, are of alkali carbonatite, whereas all of the pre-modern examples are of calcite or dolomite. Chemical and stable isotope analyses were made of separate phases of Pliocene carbonatite tuffs of the Laetolil Beds in Tanzania and of Miocene carbonatite tuffs of the Kaiserstuhl in Germany in order to understand the reasons for this major difference. The Laetolil Beds contain numerous carbonatite and melilitite-carbonatite tuffs. It is proposed that the carbonatite ash was originally of alkali carbonate composition and that the alkali component was dissolved, leaving a residuum of calcium carbonate. The least recrystallized melilitite-carbonatite tuff contains early-deposited calcite cement and calcite pseudomorphs after nyerereite (?) that have contents of strontium and barium and δ 18O and δ 13C values suggestive of incomplete chemical and isotopic exchange during alteration and replacement of alkali carbonatite ash. Carbonatite tuffs of the Kaiserstuhl contain globules composed of calcite phenocrysts and microphenocrysts in a groundmass of calcite with a small amount of clay, apatite, and magnetite. The SrO contents of phenocrysts, microphenocrysts, and groundmass calcite average 0.90, 1.42, and 0.59 percent, respectively. The average δ 18O and δ 13C values of globules (+14.3 and −9.0, respectively) fall between those of coarse-grained intrusive Kaiserstuhl carbonatite (avg. +6.6, −5.8) and those of low-temperature calcite cement in the carbonatite tuffs (+21.8, −14.9). The phenocrysts and microphenocrysts are primary magmatic calcite, but several features indicate that the groundmass has been recrystallized and altered in contact with meteoric water, resulting in weathering of silicate to clay, leaching of strontium, and isotopic exchange. The weight of evidence favors an original high content of alkali carbonatite in the groundmass, with recrystallization following leaching of the alkalies.

Ash layers from Searles Lake, California, were sampled in core of drill hole KM-3, which penetrated 693 m of lacustrine sediment deposited in a playa-lake complex over the past 3.2 my. Lake water changed from moderately saline and slightly alkaline (pH ~7.5) to highly saline 2.04 my ago and to highly saline and alkaline (pH ~9.5) 1.28 my ago. As a result of brines flushing downward, the upper 291 m of sediment, spanning the past 1.28 my, contain highly saline, alkaline pore fluid. Silicic ash layers in contact with highly saline, alkaline pore fluid were first altered to phillipsite and meriinoite and then to K-feldspar and searlesite. The transformation of phillipsite and/or meriinoite to K-feldspar required more than 45,000 years and was largely completed in 140,000 years. Tephra layers in contact with moderately saline, slightly alkaline pore fluid vary from uncemented vitric ash containing minor smectite to bentonites in which glass is wholly altered to smectite, clinoptilolite, analcime, and opal. Layers with much fine tephra are more altered than the coarser, better-sorted layers. Alteration is attributed to hydrolysis in essentially a closed hydrologic system, in which the alteration of glass to smectite raised the pH, aSiO2, and (Na+ + K+)/H+ activity ratio to the level where clinoptilolite formed. Some diffusion and/ or fluid flow is, however, indicated by the loss of SiO2 during the alteration of ash layers to smectite and by anhydrite deposited during and after clinoptilolite in some tuffaceous sandstones.

Palaeomagnetic polarity measurements from Beds I and II at Olduvai Gorge are described, and used to define the extent of the Olduvai event in the Olduvai Gorge section. The upper boundary of the event occurs very close to the base of the Lemuta Member in Bed II whilst the lower boundary is less well defined but probably occurs just below Tuff IA, The results imply minimum net accumulation rates of about 17 cm in 103 years in the lacustrine, and more than 30 cm in 103 years in the alluvial fan deposits, which represent only the upper part of the Olduvai event.

Chemical and petrographic studies of the two 720-ton ‘Colossi of Memnon’ located near Luxor, and of Egyptian quartzite quarries indicate that the rear pedestal blocks of the northern statue were quarried at Aswan, while the other, main components of the monuments were quarried near Cairo. Our studies suggest that the rear pedestal blocks of the northern statue were part of the reconstruction project undertaken by Roman emperor Septimius Severus. Our studies of the tilting of the statues further suggest that the rear part of the lower torso of the northern statue was moved when the statue was reconstructed. Part of the present-day tilting of the northern statue probably occurred prior to the reconstruction.

Remains of 13 early hominids have been found in the Laetolil Beds in northern Tanzania, 30 miles south of Olduvai Gorge. Potassium–argon dating of the fossiliferous deposits gives an upper limit averaging 3.59 Myr and a lower limit of 3.77 Myr. An extensive mammalian fauna is associated. The fossils occur in the upper 30 m of ash-fall and aeolian tuffs whose total measured thickness is 130 m.

Pellets and ooids are widespread and locally abundant in mature calcrete profiles in the Argus Range, California; near Wickieup, Arizona; and in Kyle Canyon, Nevada. Most concentrations of pellets and ooids either overlie laminar calcrete at various levels in the calcrete profile or fill subhorizontal fractures in the petrocalcic horizon. In all three profiles the petrocalcic horizon has been thickened by the pelletal, chemically deposited fracture fillings. Pellets range from 0.02 to 8.0 mm in diameter and consist principally of micritic calcite and sepiolite. Ooid coatings are chiefly calcite and opal or calcite and sepiolite. The pellets represent small concretions, some of which grew by accretion, either in void space or by displacing adjacent sediment, and the others of which were formed by cementation of pellet-shaped bodies of porous micrite. Ooid coatings with opal or sepiolite may have been deposited as a gel with sufficient strength for surface tension to thin the coatings over angular corners of nuclei so as to increase the roundness and sphericity of the particles. Major problems in calcrete genesis are (1) the cause of subhorizontal fractures and the mechanism for widening a fracture as sediment accumulates in it and (2) what determines the deposition of calcite, sepiolite, and opal as pellets and ooid coatings or as laminar layers.

The island of St. Vincent consists of an active volcano and the remains of two or more that are extinct. Late Pleistocene subaerial ash deposits forming the uppermost geologic unit over much of the island are the object of the present study. Most of the deposits are between 20 and 40 feet thick, but they reach a maximum thickness of at least 120 feet. They consist of andesitic lithic, crystal, and vitric material or their weathered equivalents. Plagioclase ($An_{50-100}$), hypersthene, augite, and olivine are the most abundant minerals. The ash was erupted from the Soufriere. The pattern of grain size and thickness suggests that it was distributed by northwesterly Pleistocene winds quite different from those of the present. Four lithologic fades have been separated on the basis of sorting, grain size, and degree of weathering-namely, the Soufriereslope, eastern-coastal, western-coastal, and upland facies. Differences between them can be explained by primary distribution of the ash and subsequent removal o...

Chert has formed from two sodium-silicate minerals, magadiite (NaSi7,O13(OH)3·3H2O) and kenyaite (NaSi11O20.5(OH)4·3H2O), in uppermost Pleistocene deposits of lakes Magadi and Natron in Kenya and Tanzania. The chert consists of finely crystalline quartz and characteristically forms nodules of irregular shape with white coatings having reticulate surface patterns. Similar nodules are widespread in lower and middle Pleistocene lacustrine deposits in the vicinity of Lake Magadi, Lake Natron, and Olduvai Gorge. Although magadiite and kenyaite are absent in the lower and middle Pleistocene deposits, the chert in these beds probably formed from a sodium-silicate precursor. All of the chert-bearing sediments were deposited in saline, alkaline lakes rich in dissolved sodium carbonate-bicarbonate. Magadiite (and chert) may form either thin, widespread deposits or localized masses which may be cross-cutting. Thin, widespread layers of magadiite have been precipitated by mixing of silica-rich brine with fresh water in a chemically stratified lake; localized masses may have been formed by interaction of brine with fresher water entering the floor or margin of the lake. Magadiite and kenyaite can alter to chert in contact with sodium-carbonate brine and possibly by leaching with relatively fresh water over a period of 20,000 years or less. The siliceous zeolites clinoptilolite and erionite predominate in trachyte tuffs associated with magadiite and chert; less-siliceous phillipsite predominates in trachyte tuffs of chert-free sequences.

A technique for the potassium-argon dating of high potassium feldspars of less than 50,000 years age is described. The technique is applied to the obtaining of high precision ages in the time-range 60,000-2,000,000 years. Sufficient data are presented to show that the time-scale of Plio-Pleistocene glaciations is greater than 10 years and that the time-scale of hominoids capable of fashioning tools by the working of stone is at least 1.75 10 years. Several other points on the time-scale of human evolution are presented. The time-scale of rift faulting in Kenya is established and the ages of several Italian volcanoes are presented.

SUMMARY Monoclinic K-feldspar, analcime, searlesite and phillipsite of diagenetic origin are present in samples of clay and tuff from U. S. Geological Survey cores of Pleistocene and Recent sediments of Searles Lake, California. Authigenic K-feldspar was identified in 37 of the 72 samples studied; analcime was identified in 21, searlesite in 8, and phillipsite in 3. Most of the K-feldspar, analcime, and searlesite occur in non-tuffaceous claystones, which form about 46% of the 875-ft. thickness of sediments that was cored. Phillipsite was found only in three rhyolitic tuff laminae within the upper 110 ft. of beds. Most of the K-feldspar and analcime crystals are less than 2 μ in diameter, and maximum diameters are 0.10 mm for K-feldspar and 0.02 mm for analcime. Phillipsite occurs as lath-shaped crystals 0.01–0.02 mm long, and searlesite principally forms spherulites 0.02–0.2 mm in diameter. Microcline, orthoclase, albite, andesine, and quartz predominate in the sand and silt fraction of the clays; illite, montmorillonite, chlorite, and kaolinite predominate in the detrital clay fraction. Grains of quartz and andesine are extensively etched at various levels below a depth of 220 ft., and only skeletal crystals of andesine remain in some clays. Volcanic glass has been completely dissolved or replaced by authigenic silicate minerals in the few tuffs. Distribution of clay minerals in the core suggests that montmorillonite and chlorite have been destroyed in many of the clays. Alkaline brine saturates the surface and subsurface sediments, and the high activity ratio of alkali ions to hydrogen ions in this brine probably accounts for the widespread formation of K-feldspar, zeolites, and searlesite. Na, K, and B for the authigenic silicates were probably supplied by the brine and by saline minerals in contact with the brine. Montmorillonite may have provided additional K. Solution of glass provided the Si and Al required to form authigenic silicate minerals in the tuffs, and solution of quartz, plagioclase, and montmorillonite most likely supplied the Si and Al for authigenic silicates in the clays.

New data show that the Olduvai normal geomagnetic polarity event is represented in Olduvai Gorge, Tanzania, by rocks covering a time span of roughly from 0.1 to 0.2 my and is no older than 2.0 my. Hence the long normal polarity event of this age that is seen in deep-sea sediment cores and in magnetic profiles over oceanic ridges should be called the Olduvai event. The lava from which the Gilsaevent was defined may have been erupted during the Olduvai event and, if so, the term Gilsashould now be abandoned. Many dated lavas that were originally assigned to the Olduvai event represent one or two much shorter normal polarity events that preceded the Olduvai event; these are herein named the Reunion normal polarity events. This revision brings the geomagnetic reversal time scale into conformity with the one implied by assumptions of uniform sedimentation rates on the ocean floor and uniform rates of sea-floor spreading.

Tuff deposits of the Koko Crater group consist largely of alkali basalt glass, either fresh or palagonitized. Most of the deposits are progressively palagonitized at depth, and topographic relations of palagonite on Koko Crater indicate that the palagonite was formed after the cone had been deeply eroded. The principal authigenic minerals in the palagonite tuffs were deposited in following sequence: phillipsite, chabazite, analcime, montmorillonite together with opal, and calcite. The amount of authigenic minerals in a given sample is generally proportional to the amount of palagonite, indicating that the authigenic minerals are produced in palagonitization of glass. Chemical analyses of sideromelane and associated palagonite by the electron microprobe show that about a quarter of the SiO2, half of the Al2O3 and MgO, and three quarters or more of the CaO, Na2O, and K2O are lost in converting sideromelane to an equal volume of palagonite. A substantial proportion of these components lost from the sideromelane are precipitated nearby in zeolites, montmorillonite, opal, and calcite. Reaction of sideromelane with percolating ground water at low temperatures accounts for the vertical zoning from relatively fresh tuffs down into palagonite tuffs. The pH and ionic strength of percolating water probably increased with depth by solution and hydrolysis of glass, and where the pH and ionic strength became sufficiently high, the glass reacted to form palagonite and zeolites. Palagonite was formed by a microsolution-precipitation mechanism rather than by hydration or devitrification.

Bed I in Olduvai Gorge spans the interval from about 1.7 to 2.0 m.y.a., and all evidences of hominid activity at Olduvai are within the latter half of this period. Bed I was deposited in a closed basin approximately 25 km in average diameter, and it can be subdivided into five lithologically different but partly time-equivalent rock assemblages, or lithologic facies, each of which was deposited in the same geographic environment or closely related series of environments. These lithofacies comprise lake deposits, lake-margin deposits, alluvial-fan deposits, alluvial-plain deposits, and lava flows.Alluvial-plain deposits form the lowermost part of Bed I, and they presumably interfinger northeastward with lake deposits in an area not now exposed. Discharge of the lavas diplaced the lake westward to a position it occupied through the latter half of the deposition of Bed I. The lake fluctuated greatly in salinity, level, and extent, and at times of low level it was highly saline and generally ranged between 7 and 10 km in average diameter. At times of high level it was as much as 15 km in diameter and was relatively fresh, at least along its southeastern margin. The lake-margin deposits accumulated on the zone of relatively flat terrain that was flooded at times of high level. Lake-margin deposits interfinger eastward with deposits of an alluvial fan of pyroclastic materials produced in an explosive phase of the volcano Olmoti. The climate at the time of Bed I was relatively dry although somewhat wetter than the present climate in the same region today.

The lower lava flow of Bed I in Olduvai Gorge, Tanganyika, carries natural remanent magnetization (NRM) having normal polarity. Thermal demagnetization experiments demonstrate the stability of this NRM. Thus the Olduvai geomagnetic polarity event, which was originally named from the upper lava flow in Bed I, is represented in its type locality by two normally magnetized lavas. These lavas have been shown to be 1.9 m.y. old, and although they are distinct from each other in composition and surface structure, their eruptions appear to have been closely spaced in time.

Andesitic glowing avalanches discharged from the Soufriere of St. Vincent on May 7, 1902, deposited ash which is noted for its fine-grained texture and richness in crystals. The largest avalanche deposit of this eruption contains several per cent of blocks and bombs and 15-30 per cent of lapilli. The median diameter in a representative sample is 0.6 mm., and the sorting coefficient is 2.9. Crystals form 45 per cent of the bombs and have been concentrated to form almost 58 per cent of all juvenile material in the deposit and 73 per cent of the juvenile ash finer than 2 mm. The glowing avalanches were discharged from the outer part of a vertical ash column which carried a large volume of ash to considerable elevations. When the column became denser than the surrounding atmosphere, it discharged clouds of pyroclastic material laterally. The clouds quickly became stratified into a lower, denser fraction (the glowing avalanches) containing the coarse ejecta and bulk of the crystals, and an upper part lighter a...