The Western San Juan Mountains: Their Geology, Ecology, and Human History

5. Mesozoic and Early Cenozoic History

CHAPTER 5 MESOZOIC AND CENOZOIC HISTORY JOHN A. CAMPBELL AND DOUGLAS C. BREW The Mesozoic Era lasted trom about 245 million to 66 million years ago. It, like the Paleozoic, is divided into shorter intervals called periods. The periods of the Mesozoic are the Triassic, Jurassic , and Cretaceous (see Stratigraphic Chart, p. 2). These periods are not of equal length: the Cretaceous is the longest, about 78 million years, and the Triassic the shortest, about 37 million years. By looking again at the Stratigraphic Chart, one can see that Mesozoic formations, like those of the Paleozoic, include unconformities , with much of the Triassic and some of parts ofthe Jurassic missing. These breaks in the geologic record are due to erosion, nondeposition, or a combination of the two. The geologic history ofthe San Juan Skyway area during the Mesozoic provides an interesting contrast to that ofthe Paleozoic. Whereas in the early Paleozoic deposition ofsediments took place in shallow, warm seas, during the first part of the Mesozoic this area was subjected to periods of weathering and erosion and deposition by streams, lakes, or wind. In the latter part of the Mesozoic , sediments were deposited in a sea that covered much of central North America, whereas in the later Paleozoic the sediments were deposited by streams at the foot of an ancient mountain chain. We have divided the following discussion into two sections to reflect the two depositional styles evident in the Mesozoic geology of the San Juans region. 54 Mesozoic and Cenozoic History 55 EARLY MESOZOIC GEOLOGIC HISTORY Weathering and erosion of the landscape, particularly of the Ancestral Rocky Mountain chain that formed in late Paleozoic time, continued into the early part ofthe Mesozoic. No deposits ofearly Triassic time are known in the Skyway area (MacLachlan 1972). The oldest preserved deposits are those ofthe late Triassic Dolores Formation (Fig. 5.1, Points of Interest 14). This formation was deposited by streams, lakes, and wind (Blodgett 1984). The resulting sandstones, shales, and siltstones are typically oxidized ("rusted") to a bright red and thus are difficult to distinguish on the outcrop from the underlying Permian Cutler Formation. However, an angular unconformity exists between the Cutler and the Dolores in many places; it can best be seen in the Ouray area (Weimer 1980). This unconformity indicates movement along faults in the northern side of the Skyway area during late Permian or early Triassic time. Weathering and erosion again dominated following the creation ofthe Dolores Formation. This interval continued through the early Jurassic, by which time all vestiges of the Ancestral Rocky Mountains were gone. The Skyway area was now low, flat, and probably not very heavily vegetated. A desert was starting to form (Peterson 1972), and the wind blowing across the area deposited sand in large dunes. Cross-bedding in these sandstone beds is highly variable but indicates a wind that blew from the northwest toward the southeast. This descrt was very large, covering a vast area ofthe western United States (Peterson 1972). Later the sand was cemented together in a sandstone called the Entrada Formation (Fig. 5.1). North and perhaps west of the Skyway area there was a sea, the beaches of which may have been the source for the sand in the dunes. The Wanakah Formation, which overlies the Entrada (Fig. 5.1, see Points ofInterest 10, 80, 93, 131), comprises shales, siltstones , and a few sandstones and limestones, all ofwhich were deposited in or near a large, shallow body of water. Some geologists think this body ofwater was a lake or playa that eventually covered the dunes in the Skyway area. Others think this may have been a shallow sea. No fossils or other clear indicators have been found to 56 • The Western San Juan Mountains o 5 '" o '" w ~ Upper Cretaceous lower Cretaceous Juta. $$ic: Mancos Shale Dakota Formation ~Canyon FonnaUon Brushy - , Basin Member Salt Was!> Member MORRISON FORMAnON Junction Creek FormaHon --- - .. -----1 YVanakahFonnaUon nI'MS10ne Entrada FormaUon ----_ ----1 Dolores Formation 6 - - - - ------; Wu ...1 - .-> >Lewis Sh. Older Cretaceous Units V.E. "" 3.8 X 2500 m 2000 1500 1000 Fig. 5.3 Diagrammatic cross-section showing stratigraphic relationships of late Cretaceous and Tertiary units. View from U.s. 50 south of Durango (modified after Condon, 1990). S: ~ o n' o " Q. ~ K I 0w 64 The Western San Juan Mountains OLDER UNITS The oldest...