QUATERNARY GEOLOGY OF THE PLAIN CITY 7.5' QUADRANGLE, UTAH

The surficial geology of the Plain City quadrangle contains a variety of geomorphic features that originated from fluvial, lacustrine, mass-movement, and tectonic processes. The quadrangle takes its name from Plain City, a small rural community with a population of 4,100 in Weber County about 60 km north of Salt Lake City, Utah. Plain City lies atop a remnant of an early Holocene fine-grained delta (highest elevation ~ 1,292 m) formed by the Weber River as it flowed into post-Gilbert level Great Salt Lake between about 9.7 and 9.4 ka. The Plain City delta stands about 8 m above the modern flood plain of the Weber River and the northwestern boundary of Plain City forms a well-defined, sublinear late Holocene shoreline at 1,287 m elevation that is traceable for about 4.8 km along the ground surface. Formed between 2.5 and 2.0 ka, remnants of this shoreline have been identified elsewhere in the Lake Bonneville basin and are considered by many researchers to mark Great Salt Lake's late Holocene high water level. Modern-day Great Salt Lake lies in the northwestern corner of the quadrangle.

Quaternary tectonic processes have shaped much of the land surface in the eastern part of the Plain City quadrangle; the degraded and modified toe and lower portion of a large, 25 km² liquefaction-induced lateral spread/flow failure complex (the North Ogden landslide complex) occupy about 6 km² of the eastern part of the quadrangle. Geomorphic evidence shows that the landslide complex consists of both lateral spreading and flow failures. Radiocarbon age estimates show that it likely formed in multiple earthquakes during early to late Holocene. Liquefaction-induced landsliding is most likely related to earthquakes on the nearby active Wasatch fault. Holocene fault scarps are present in the northeastern corner of the quadrangle as is the Pleasant View salient, which defines the boundary between the Weber and Brigham City segments of the Wasatch fault.