A debris avalance deposit forms a 300-km²-lobe about 27 km long and up to 18 km wide south of Escuintla. The greater part of the avalanche deposit was previously mapped as Tertiary volaniclastics of the Escuintla love, and several isolated parts were mapped as Tertiary volcanics (Hunter et al., 1984). The morphology, characteristics, and stratigraphic relations of the unit indicate that it is a late Pleistocene debris avalanche, herein called the Escuintla debris avalanche.
The Escuintla debris avalanche can be traced 2 km north of Escuintla where a small island of hummocky terrain is surrounded by younger alluvial fan deposits. Two other deposits are isolated from the west margin of the Escuintla fan. The Guacalate River system has eroded some of the avalanche deposit at its northwest margin. The avalanche was not positively identified more than 2 km upstream of Escuintla.
The characteristic morphology of the avalanche includes a slightly convex cross-sectional profile and numerous hummocks varying in height from 5 m up to about 50 m. The avalanche has resulted in a reversal of topography from that which existed at the time of its occurrence and major rivers now flow just beyond its distal margins. Along the apex of the avalanche lobe, there are alternating zones having higher and lower hummock densities. Between the apex of the lobe and 10 km down slope and between 2 and 5 km from the lateral and distal margins of the deposit, the size and density of hummocks is greater. Areas within 1 to 2 km of distal margin and in the middle of the lobe have smaller hummocks and lower hummock densities. Debris avalanches commonly exhibit decreasing hummock size and density with increased distance from the source. The alternating distribution of hummocks within the Escuintla debris avalanche might have been the result of two (or more) slide blocks--a first larger one that came to rest at the distal margin and a second that came to rest about 10 km south of Escuintla.
The hummocks contain huge blocks of basalt, basaltic andesite, and rare andesite breccia and lava, which is commonly fractured, deformed, and faulted. In hummocks that contain several lithologies, complexly folded contacts and faults between breccia masses, commonly exhibit breccia schlieren and large-scale (2 to 20 m) texture and color variations. Matrix, which is well exposed only at the flanks of the hummocks, comprises angular to subangular pebbles, cobbles, and boulders of basalt and basaltic andesite in silt and sand. The matrix contains incompletely mixed zones of variable colored breccia, which form color variations on a small scale (<1 mm).
The thickness of the debris avalanche is not well known. The margins of the avalanche fan, however, exhibit approximately 20 m of relief in comparison with adjacent alluvial surfaces, suggesting at least 20 m of thickness. Moreover, comparison of the two adjacent river plains with the medial axis of the Escuintla fan indicates at least 60 m of relief. Using 30 m as a conservative estimate of the average thickness indicates that the avalanche has a total volume of at least 9 km³.
Because of its position between Río Michatoya and Río Guacalate, the Escuintla debris avalanche could have originated from Agua, Acatenango, or Fuego volcanoes. Several lines of evidence indicate, however, that the avalanche came from the Fuego-Meseta complex. First, the predominant basalt, basaltic andesite, and minor andesite of the debris avalanche more closely match the composition of Fuego and Meseta (the remnant of an old cone upon which Fuego is built) than of the more andesitic volcanoes Agua and Acatenango (Chesner and Rose, 1984). Second, no collapse scar is now visible on Acatenango or Agua Volcanoes but Meseta exhibits a prominent scarp partly covered by the younger Fuego Volcano. Third, the Escuintla debris avalanche fan is positioned east of the center of the complex of depositional fans south of Escuintla; thus the debris avalanche would have had a straighter path between its source and its position on the coastal plain if Fuego, Meseta, or Acatenango rather than Agua were the source volcano.
The debris avalanche is older than a 2,000-year old lahar of Hunter et al. (1984) that occupies the flood plain of the Guacalate River and soil development on the deposit varies from 2 m thick at the top of hummocks to as thick as 5 m at the base of hummocks, indicating an age that ranges to 10's of thousands of years. Soils typically comprise 1 to 2 m of red laterite and about 2 m of weathered avalanche debris. We suggest that the avalanche postdates the 85 ka Los Chocoyos ash. The Los Chocoyos ash was never observed to overlie the avalanche deposit even thought the ignimbrite crops out only 12 km west of Escuintla. Moreover, another ignimbrite, which is either Los Chocoyos or an Amatitlán unit crops out about 5 km north of the avalanche deposit near the mountain front. By comparing eruption rates to edifice volumes Chesner and Rose (1984) suggest that a minumum of about 8,500 years were required to build Fuego volcano. Moreover, they suggest that the Fuego-Meseta complex is probably not more than 30,000 years old. If the Escuintla debris avalanche had an origin at Meseta, then its age is inferred to be between about 8,500 and 30,000 years.
(Vallance et al., 1988)