PROGRESS TOWARDS IMPROVED STRATIGRAPHIC NOMENCLATURE AND A NEW STATE GEOLOGIC MAP FOR OREGON

We are engaged in two parallel efforts to make geologic mapping and stratigraphy within the Oregon Geologic Data Compilation (OGDC) more accessible. OGDC is a variable-scale collage of the best-available geologic mapping within the state. The current version, OGDC-7, mosaics 342 source maps with 7,240 source-map units. Source-map units are classified into a terrane/group – formation – member – unit hierarchy and assigned values of rocktype, age, and lithology; this results in more than 2,000 unique compilation units.

Our first effort is a DOGAMI-USGS co-op to establish regional and statewide stratigraphic correlations and produce time-rock charts. We reduce the terrane/group – formation – member – unit hierarchy to about 540 unique formation-level names statewide. This includes assigning formation-level names to source-map units that are classified only at the terrane/group level in OGDC-7. Some names are formal, some are informal. We are building time-rock charts at the formation level for NW Oregon, SW Oregon, the Cascade Mountains, NE Oregon, and SE Oregon, and a single chart at the terrane/group level for the state. Primary goals are to depict stratigraphic relations within OGDC and to identify needed updates to Geolex, the USGS-maintained lexicon of stratigraphic names.

Our second effort is to assemble these formation-level units into about 100 units suitable for depicting the geology of Oregon at 1:500,000 scale. We start by recognizing 9 fundamental suites: rocks and deposits of the modern (Cascade) regime, rocks of the early to middle Miocene Yellowstone hotspot, rocks of Siletzia, rocks of the Challis (Clarno)-Cascade transition, rocks of the Challis regime, rocks of the coastal Gold Beach terrane which appears to have been accreted in the early Cenozoic, little-deformed mid- and Late Cretaceous strata, rocks of the Nevadan orogeny, and older accreted terranes. These suites are further subdivided by age, genesis, and composition. We have chosen not to divide rocks of the modern regime into arc and back-arc assemblages. Our goal, after much yet-to-be-accomplished generalization, is a new geologic map for the state of Oregon.