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chapter 2 Volcanology, Stratigraphy, and Effects on Structures C. Dan Miller Introduction Geological and volcanological studies at the Cerén site were designed to provide a stratigraphic framework for archaeological and other investigations at the site, to provide information about the character of the eruptions that destroyed and buried structures at the site, and to provide details about the source and distribution of volcanic deposits that mantle the site. Stratigraphic sections were measured and described in excavations at each of the main structures at Cerén (Fig. 1.1) to reconstruct the sequence of eruptive events and to allow comparison of the sequence of deposits from one structure to the next. Relationships between stratigraphic units and roof thatch and walls were noted to determine the timing of the destruction of structures during the eruption. The character and thickness of deposits preserved inside of structures varied greatly from undisturbed sections outside, and were dependent upon the timing of damage to walls and roofs. At each site, stratigraphic units were sampled and textural and granulometric characteristics were analyzed . In addition to excavations at the Cerén site, more than forty distal sections of the Cerén sequence were examined to determine the distribution and thickness of deposits and to produce an isopach map. I gratefully acknowledge assistance in the field by Brian R. McKee and Eduardo Gutiérrez. I thank Marvin Couchman, U.S. Geological Survey, for doing sieve analyses of eruptive units at Cerén. Chemical analyses of Cerén deposits were performed by David Siems, U.S. Geological Survey. Origin and Character of the Cerén Sequence geologic setting Excavations at Cerén have exposed a uniform series of pyroclastic deposits slightly more than 5 m thick (Fig. 2.1). The sequence sits on up to 50 cm of tierra blanca joven (TBJ) tephra, a distinctive whitish dacite tephra that erupted about a.d. 260 (Hart and Steen-McIntyre 1983) during the catastrophic eruption of Ilopango Volcano, about 40 km to the southeast . Nearly 5 m of the Cerén sequence is derived from eruptive source(s) within about 1.4 km of the Cerén site. Near the top of the Cerén sequence are tephras inferred by Hart (1983) to have come from eruptions of nearby Boquerón and Playón. The bulk of the Cerén sequence was produced by eruptions that occurred at one or more vents within a distance of about 1.4 km north and east of the Cerén site, as suggested by Hoblitt (1983) and Miller (1993). The vents lie along a fissure that extends in a north-northwest direction from San Salvador Volcano (Zier 1983). Historically, the fissure has been the locus of several eruptions during the past several thousand years, along a line of vents between Laguna Caldera Volcano and the north flank of Boquer ón (San Salvador) Volcano (Fig. 2.2). Magmas from the fissure are basaltic andesite with compositions of about 56% silica (Table 2.1). Magmas that erupted from some parts of the fissure had no signi ficant interaction with surface water or ground- 12 c. dan miller figure 2.1. Loma Caldera eruptive sequence at Cerén site. Partially excavated Structure 1 is in middle of photograph. The Cerén sequence in this picture is approximately 5.2 m thick. Units 1 and 3, near the base of the section, vary in thickness in the vicinity of Structure 1. The walls of Structure 1 were oxidized where they were in contact with Units 1 and 3. Photograph by Payson D. Sheets. table 2.1. Chemical Analyses of Cerén Rocks Ballistic Lapilli Lapilli Bomb Lava from Oxide of Unit 2 of Unit 4 in Unit 2 El Playón SIO2 . . . . AL2O3 . . . . FETO3  . .  . . MGO . . . . CAO . .  . . NA2O . . . . K2O . . . . TIO2 . . . . P2O5 . . . . MNO . . . . LOI C − .  − . − . — TOTAL . . .  . note: Whole rock XRF analyses of lapilli from Units 2 and 4 and a ballistic block associated with Unit 2 at the Cerén site, and a whole rock emission spectrometry analysis of lava from Playón Volcano (Hart 1983). Analyses of Units 2 and 4 and the Unit 2 ballistic block completed by David Siems, U.S. Geological Survey. FeO and Fe2O3 reported as total iron, FETO3. LOI 925C values represent oxidation of sample and loss of volatile components during analysis of sample at 925°C. water, as, for example, the eruptions at Laguna Ciega, the Playón lava flow and cinder cone that erupted in a.d. 1658, and the eruption of...

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