CRETACEOUS AND TERTIARY PALEODRAINAGE IN THE SOUTHERN INTERIOR PLATEAU, SOUTHWESTERN BRITISH COLUMBIA

Physiographic mapping of high-resolution DEM-based shaded relief maps delineates landforms and drainage networks in the southern Interior Plateau from 49°N to 52°N. Geomorphology, valley-fill geology and cross-cutting relationships with dated geologic structures indicate a suite of early Tertiary and older valleys are preserved in the modern landscape. The correlation of physiography with sedimentary paleoenvironment suggests that some landform elements are exhumed features of middle Cretaceous age (ca. 100 Ma). Landscape reconstructions show a long-lived north-flowing drainage pattern during Cretaceous and Eocene time fed from headwaters in the Coast Mountains, Thompson and Okanagan Highlands. Cretaceous base level was more than 1000 m above modern sea level and is preserved in elevated tracts throughout the study area. Eocene drainage was incised into this surface. Base level during the Eocene was a minimum of 400 m elevation and dipped gently northward. Volcanism and faulting in Eocene time disrupted drainage in places and caused ponding. During Oligocene and Miocene time, base level was some 500 m higher and Eocene valleys were partially buried. Miocene rivers in the Coast Mountains flowed eastward across the Thompson Plateau and northward along Clearwater and Quesnel river valleys. Not until the Pliocene did the modern south-flowing Fraser, Thompson, Chilcotin and Okanagan drainages develop. Preservation of relict and exhumed landforms of Eocene and even Cretaceous age suggests that Quaternary glacial erosion was not extensive on a regional scale.