Abstract The Plio-Pleistocene sedimentary deposits of upper Bed I and lowermost Bed II in the Olduvai basin, Tanzania, are analyzed in terms of sedimentary processes, paleoenvironments, and paleoclimate. The sedimentary basin was a shallow, closed depression (∼ 50 km wide with strata ∼ 100 m thick) located adjacent to the East African Rift between Precambrian basement rocks to the west and the large Plio-Pleistocene Ngorongoro volcanic complex to the east. Olduvai lies on the west flank of the main rift valley, and this rift platform area was subjected to crustal extension resu I ting in normal block faulting and block tilting. Through time the axial part of the sedimentary basin episodically migrated eastward toward the main rift valley. Syndepositional volcanism included basalt flows, pyroclastic air falls, ground surges, and ignimbrites from eruptive centers of the Ngorongoro volcanic complex. Hay (1976) interpreted the paleolandscape to have had a central lake basin that was filled with detrital sediment, carbonate rocks, and tephra. Detrital sediment was provided both by rivers sourced in the Ngorongoro complex and by Precambrian rocks of the Serengeti to the north and west. Recent high-resolution stratigraphic studies of paleoenvironments in a time slice (1.85-1.75 Ma) found that the sedimentary facies vary temporally and spatially because of the interplay among five independent variables affecting the sedimentary record: (1) sediment flux (pyroclastic air fall, intermittent fluvial runoff and mudflows, and eolian), (2) sediment composition (Precambrian source rocks vs. volcanic source rocks), (3) hydrology and chemistry (lake water, surface water, and groundwater), (4) soil formation, and (5) tectonics associated with rift extension. Lake cycles, ephemeral fluvial systems, soil formation on the interfluves and lake margin, well as a large groundwater-fed wetland (> 3 km 2 ) are all part of the Olduvai rift-margin basin record and provide insights into the Plio-Pleistocene paleoclimate history of East Africa.

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Howel Williams, known to all of his close friends as “Willie,” has inspired a generation of students and a host of professional colleagues through his enthusiastic devotion to the study of volcanoes. Willie, an identical twin of Welsh ancestry, was born October 12, 1898, in Liverpool, England. Toward the end of the First World War, he served with a survey unit of the Royal Engineers, and later, while resuming his interrupted course in geography at the University of Liverpool, his interest was kindled first in archeology and then in geology. According to legend, the fossilized trackway of a Triassic reptile imprinted on a large slab of local sandstone on display in the Geology Department fired his imagination and helped to lure him into the paths of geology. Clambering as a youth among the volcanic rocks of his parental county of Caernarvonshire in North Wales proved to be a prelude to his initial ventures into geological research, first on the igneous rocks of the Capel Curig district and then on the geology of the nearby mountainous tract of lower Paleozoic rocks centered on the peak of Snowdon. His classic account of The Geology of Snowdon (1927) was acclaimed especially for its lucid descriptions of the sequence, structure, and petrology of the Ordovician volcanic suite, and for the excellence of his mapping. It was in Snowdonia that he acquired an abiding love of field geology. This introduction to volcanic geology encouraged Willie to come west for three years as a Commonwealth Fellow

Williams, Howel, 1921, Excavations of Bronze Age Tumulus, near Gorsedd, Holywell, Flintshire: Archaeologia Cambrensis, 1921, p. 265–289. ——— 1922, Fish-bone from the Gorsedd Tumulus, Holywell: Archaeologia Cambrensis, 1922, p. 150–152. ——— 1922, The igneous rocks of the Capel-Curig district, North Wales: Liverpool Geol. Soc. Proc., v. 13, p. 166–202. Williams, Howel, and Williams, David, 1924, Easter excursion to Snodonia: Liverpool Geol. Soc. Proc., v. 14, p. 12–15. Williams, Howel, 1926, Notes on the characters and classification of pyroclastic rocks: Liverpool Geol. Soc. Proc., v. 14, p. 223–248. ——— 1927, The geology of Snowdon (North Wales): Geol. Soc. London Quart. Jour., v. 83, p. 346–427. ——— 1927, Kilauean ashes: Volcano Letter, no. 125. ——— 1928, A recent volcanic eruption near Lassen Peak, California: California Univ. Pub., Dept. Geol. Sci. Bull., v. 17, no. 7, p. 241—263. ———1929, Age of the Tahitian coral-reefs: Nature, v. 74, p. 727–728. ——— 1929, Geology of the Marysville Buttes, California: California Univ. Pub., Dept. Geol. Sci. Bull., v. 18, no. 5, p. 103-220. ——— 1929, The volcanic domes of Lassen Peak and vicinity, California: Am. Jour. Sci., 5th ser., v. 18, p. 313–330. Greenly, Edward, and Williams, Howel, 1930, Methods in geological surveying: London, Thomas Murby and Co., New York, D. Van Nostrand Co., 420 p. Williams, Howel, 1930, Notes on the later geologic history of Tahiti (Society Islands): California Univ. Pub., Dept. Geol. Sci. Bull., v. 19, no. 5, p. 119–135. ——— 1930, The Snowdon district—Report of the Easter field meeting, 1930:

Silicic volcanic rocks — dacite, rhyolite, and quartz porphyry — constitute about 35 percent of the Yavapai Supergroup, an older Precambrian sequence in central Arizona. In addition, the series contains about 30 percent pillow and amygdaloidal basalt, 5 percent andesitic rocks, and the remainder is mixed andesitic and silicic-bedded tuffaceous rock. The Yavapai Supergroup is divided into the Ash Creek and Alder Groups, each containing about 20,000 feet of lavas and pyroclastic rocks; no evidence is available to determine the relative ages of the two groups. The pillow basalts, coarsely graded volcanic breccias, and poorly sorted and graded tuffaceous-bedded rocks suggest a marine accumulation in an eugeosyncline. The Yavapai Supergroup is metamorphosed to the greenschist facies. The Ash Creek Group exhibits open folds but is nonfoliated except locally, whereas, the Alder Group is isoclinally folded and dominantly schistose. Twenty-five chemical analyses of the metamorphosed, silicic volcanic rocks reveal varying ratios of sodium and potassium. By comparison with young rhyolitic obsidians, only two rhyolites approach the ratios and total alkali content found in recent obsidians. Five rhyolites contain more sodium and appreciably less potassium, whereas none of the rhyolitic rocks contain more potassium than in young obsidians. High-sodium content is reflected by much albite, and high-potassium content by abundant sericite. Iron-magnesium metasomatism locally has produced much chlorite associated with an increased quartz and decreased albite content. The source of the iron and magnesium may be underlying pillow basalts. Many rhyolitic rocks of Phanerozoic age display variable ratios of Na 2 O and K 2 O caused by hydration of obsidian, hot spring action, and hydrothermal solutions. The redistribution of the alkalis in the Yavapai Supergroup may be caused in part by factors other than regional metamorphism. It is suggested the H 2 O + content, averaging between 5 and 6 percent by volume in these metamorphosed silicic rocks, may be derived in part from interstitial marine water.