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ATTACHMENT F:MAPS AND CROSS SECTIONS OF GEOLOGIC STRUCTURE OF AREA west). The Thornton arch is an east-west to northwest-southeast trending anticlinal feature located almost 10 miles north of the site,representing the structural crest located up-dip from STIG #1. The Stockton Fault,found 12 miles south of the STIG-LEC facility,is an Eocene cross-valley feature that separates the relatively down-dropped Delta area from the northern part of the San Joaquin Valley. The Midland Fault Zone,located 14 miles west of the STIG-LEC facility,represents a zone of west-dipping normal faults that ceased active movement more than 30 million years ago in the area. Regional Stratigraphy The eastern shelf of the Southern Sacramento Valley has a basement complex composed of Sierran granitics that intrude Jurassic metasediments and metavolcanic rock units. Immediately overlying the basement rocks of the Lodi area is a 5,500-foot-thick series of Upper Cretaceous marine sedimentary units that are the accumulated detritus derived from eastern Sierran upland sources. This sequence began with relatively deep-water Guinda and Forbes turbidities;continued with a progressively shallowing(regressive) marine- deltaic-fluvial environments;and ultimately ended with uplift, subaerial exposure,and limited erosion. Unconformably overlying the Cretaceous section is a series of five Eocene formations that have a total thickness of about 2,500 feet in this region. The Meganos and Capay shale units represent two distinct episodes of transgressive gorge and deltaic fill,followed upward by deposition of the mature,well-sorted marine Domengine sands,a deeper environment unit of Nortonville Shale, and a shallowing marine sequence of Markley thin sand-siltstone-shale interbeds. Non-marine continental deposits that range in age from Oligocene to Recent represent the most near-surface 2,750 feet of stratigraphic section found in the region. This stratigraphy is summarized in Figure F-2. Regional Tectonic History The tectonic history of the eastern shelf of the Southern Sacramento Valley is fundamentally related to its initial setting as a fore-arc basin. The deposition of the thick sequence of Cretaceous marine sediments was contemporaneous with magmatism and uplift in the Sierra Nevada, and with deposition and deformation of the trench-filling Franciscan Complex located to the west where the Coast Ranges now exist. By the end of the Cretaceous,this eastern shelf area was tilted to the south and west,uplift to the west had formed a broad ridge essentially damming the basin from the ocean,and mild regional deformation had initiated formation of the faults and folds in the area. Intermittent periods of uplift and subsidence affected the depositional basin throughout the Early Tertiary, and by the end of Oligocene time,the convergent arch-trench system was replaced by the San Andreas transform fault system that operates at present. This system of near-vertical,right lateral displacement faults passes through the San Francisco Bay Area 50 miles west of the Lodi area,as it extends from the Imperial Valley of Southern California to Cape Mendocino. Following the development of that San Andreas system,the only significant geologic activity to occur within the eastern shelf region of the Southern Sacramento Valley has been the gradual filling of the basin by detritus derived from the western slope of the Sierra. F-2 SAC/371322/082550021(ATTACHMENT-F.DOC)