USING CHANNEL PLANFORM GEOMETRIES TO IDENTIFY TIDAL CHANNELS ON RELICT LANDSCAPES: A NOVEL APPROACH TO TRACKING SEA LEVEL CHANGE

Quantitative analysis of channel planform geometries offers a powerful tool for reconstructing past sea level, particularly in tectonically active arid coastal regions where crustal uplift results in relative sea-level fall and preservation of ancient tidal channels on relict landscapes. This approach requires reliable criteria to distinguish tidal channels from fluvial channels, a goal made difficult by the overlap of metrics such as curvature, sinuosity, and meander wavelength for the two channel types. The asymmetry index does show statistically significant difference between tidal and fluvial meanders in global datasets, but by itself is not suitable for testing the origin of individual channels. To improve the range and utility of existing metrics, and explore the potential for identifying paleo-sea-level indicators, we conducted an analysis of modern and relict channel networks to identify unique patterns in planform geometries and evaluate their applicability for classifying tidal versus fluvial relict channels. Preliminary measurements of modern systems reveal distinct, quantifiable differences in along-channel curvature, width, and stream order jumps for the two channel types. Tidal channels display a pronounced down-channel increase in channel width, systematic decrease in along-channel curvature, and large closely spaced jumps in stream order at channel junctions. In contrast, fluvial channels display relatively constant channel width, inconsistent changes in curvature, and smaller more widely spaced stream-order jumps. These patterns represent diagnostic criteria that can be used in conjunction with existing published metrics to classify channels of unknown origin. We are using this approach to test the tidal versus fluvial origin of relict channels on uplifted landscapes in central Baja California peninsula (Mexico) and the leading edge of the Zagros thrust belt in coastal Iran. Preliminary results indicate the presence of ancient tidal channel networks preserved on relict landscapes at elevations of ~100 to 300 meters above modern sea level, with estimated ages ranging from ca. 100 ka to 3-4 Ma. These findings highlight the promise of this approach for classifying tidal versus fluvial channels as a useful tool for reliably identifying paleo-sea level indicators.