OCR Text |
Show Basin north of Las Vegas has a high average elevation of about 1.5 km, the same as the average for the Colorado Plateau, so overall collapse did not cause the Great Basin. At the same time, the western Snake River Plain sagged to form several large lakes and the southern Great Basin was differentiated by maintaining low elevations south of several strike slip- faults ( Garlock and Lake Mead). During this period of time, the chief attributes of the modern Great Basin developed: Mountain blocks averaging 25 to 35 km spacing, each generally 15 km wide; gentle to steep tilts on young sediments indicating faulting causes block tilting; high heat flow and thin ( 30 km) crust. Thus, internal drainage with little integration of fluvial systems from one fault block to another combined with severe rain shadows from the Sierra Nevada and Transverse Ranges to provide the modern Great Basin hydrologic and climate systems. Conclusions Geology affects the development of lakes by direct and indirect effects of many kinds. Most evident is the immediate control of micro- climates through tectonically- driven topography that affects river distribution, altitude ( and thereby snow and ice accumulation) and rain shadows. Other controls on lake development include nutrient availability from rock and soil, development of local landforms, and distant geologic influences on global climate. Lakes present in the Great Basin throughout the last 100 million years formed in many geologic environments. Lakes have occupied small collapse basins near the crests of high- altitude Andean- type mountains, larger depressed orogenic basins within mountains, and basins of varying sizes in rapidly extending crust. Each geologic environment has predictable and testable implications for aquatic physics, chemistry, and biology. The fossil record holds great promise for evaluating the physical and chemical environment of these past lakes. The geologic record suggests that a few large lakes existed during the Cretaceous to Oligocene, whereas many smaller lakes unique to the Great Basin may have existed during and after the Miocene. Study of fossils from these lake sediments could establish to what degree these basins were interconnected. The Great Basin achieved its present topography, and therefore the approximate distribution of modern lake basins, roughly 3 million years ago. Since the region has been semi- arid within the rain shadow of the Sierra Nevada much of that time, it probably was also internally drained during much or all of this time period. Intermittent lakes of the Great Basin probably have a fragmentary depositional record of this time period, but the record may be relatively complete in Great Salt Lake and in several lakes along the east margin of the Sierra Nevada, such as Mono, Walker, and Pyramid Lakes. Understanding the depositional record of earlier lakes- those present in the early Pliocene and Miocene- can be a fruitful research topic to shed light on the ~ 9 to 5 Ma event that ushered in the modern Basin and Range province. |