Culture and Ecology of Chaco Canyon and the San Juan Basin

22 Chaco Project SynthesisGeologyStudies of the bedrock geology and paleontologyof the Upper Cretaceous (Siemers and King 1974)expand on Schalk and Lyons's description of ChacoCanyon as a contact zone. Upper Cretaceous marinesediments are found in the walls of the canyon, andalso to the north, where are found the Allison Member(shales); the Mesaverde group (which contains PointLookout Sandstone, the Menefee Formation, and CliffHouse Sandstone); Lewis Shale; Pictured CliffSandstone; Fruitland Shales; and the Ojo Alamo andPuerco formations. Badlands, which have littlevegetation, appear in shale deposits. Brand (1937c:55-63) reported that barite, gypsum, aragonite,siderite, and petrified wood are associated withKirtland Shale. Silicified wood, pebbles of redjaspery quartz, brown and gray chert, vein quartz,pink and white quartzite, rhyolite, andesite, felsite,porphyrite, granite, gneiss, schist, and obsidian werefound in the Ojo Alamo Shales. This formation alsocontains limonitic and manganese concretions. Calcitecrystals are present in the Puerco Formation, and theTorrejon Formation contains chert pebbles and quartz.There is a wealth of stone that could be used forconstruction and tool manufacture, as well as forfashioning ornaments and grinding pigments. Alsowithin the Fruitland Formation, various Dinosauria,Chelonia, and Pices fossils occur (Brand 1937c:40-41).In the canyon, Cliff House Sandstone is thepredominant stratum that forms the north wall. Fossilremains in the walls include species belonging to theclasses Gastropoda, Pelecypoda, and Cephalopoda,especially Inoceramus barabini and sharks' teeth(Brand 1937c:40-41; Vann 1931,1942). On the northside of the canyon, the cliff wall rises approximately20 to 30 m to a bench containing shale before risingagain to full height of 85 to 95 m above the canyonbottom (Siemers and King 1974). Several re-entrantscut the cliff and funnel water into the canyon duringinfrequent rain storms. Water from the top of theNorth Mesa, however, flows toward the EscavadaWash. On the south side of the canyon, Chacra Mesais from 91.5 to 152.4 m high, dips to the north, andis broken by several gaps (e.g., Vicente Wash, whichenters the Chaco near Fajada Butte and South Gap).These shallow drainage systems move alluviumintermittently toward the Chaco Wash. This combinationof runoff from re-entrants from the northerncliff walls and the southern washes brings additionalwater to the canyon bottom that helps alleviate someconstraints in an environment that is covered withsparse vegetation and receives minimal rainfall foragricultural production (Gillespie 1985; R. Powers etal. 1983; Schelberg 1982a; Gwinn Vivian 1990).Four species of fossils recorded by Siemers andKing (1974; see also Vann 1931, 1942) indicate minorregressions and transgressions of the UpperCretaceous shoreline in Chaco Canyon. Some fossilsare bivalves found today in depths of marine waters;others are found only in shallow marine environments;a few represent intertidal zones. The several freshwaterand brackish-water gastropods suggestproximity to a m~or fresh-water influx and estuarineconditions. Siemers and King interpreted this torepresent changes in the shoreline of an ancient lake.They concluded that the Cliff House Sandstone wasprobably a one-time barrier beach front during ashoreline standstill before it migrated farther south.Siemers and King (1974:270) also examined theclay mineralogy of the Menefee Formation, CliffHouse Sandstone, Lewis Shale, and Pictured CliffsSandstone. Clay from the Lewis Shale contains calcareousconcretions; the minerals are predominantly aNa-montmorillonite, with some illite, a mixed layer ofillite-montmorillonite, and a little kaolinite. Concretionsin the Menefee Formation include siderite andcaIcite- and barite-bearing materials. D. Love(1977b:Table 11; 1980) would use these data tocharacterize soil composition that originates in theupper wash versus that from side washes draining intothe canyon. Understanding the Chaco Basin, therefore,is relevant to understanding Chaco Canyon.To initiate studies of the Chaco Basin,DeAngelis (1971, 1972) assessed the availableliterature and found that 18 studies were morepertinent for a regional overview than specific to theregion. He divided the Chaco Basin into 19 subbasins(Figure 2.1) and examined the drainage pattern on alarge scale. He found no true dendritic pattern aswould be expected in a hasin developed on nearly flatlying,sedimentary rocks. Instead, there is considerableevidence for structural control, and threeanomalies become apparent: