Contingency table analysis of pebble lithology and roundness: A case study of Huangshui River, China and comparison to rivers in the Rocky Mountains, USA

Contingency table analysis of pebble lithology and roundness is an effective way to identify the source terrane of a drainage basin and to distinguish changes in basin size, piracy, tectonism, and other events. First, the analysis to terrace gravel deposited by the Huangshui River, northeastern Tibet Plateau, China, shows statistically contrasting pebble populations for the oldest terrace (T7, Dadongling, 1.2 Ma) and the youngest terraces (T0–T3, ≤ 0.15 Ma). Two fluvial processes are considered to explain the contrast in correlation between lithology and roundness in T7 gravel versus T0–T3 gravel: 1) reworking of T7 gravel into T0–T3 gravel and 2) growth in the size of the river basin between T7 and T0–T3 times. We favor growth in basin size as the dominant process, from comparison of pebble counts and contingency tables. Second, comparison of results from Huangshui River of China to three piedmont streams of the Rocky Mountains, USA highlights major differences in source terrane and history. Like Rocky Mountain piedmont gravel from Colorado examples, the Huangshui gravels show a preference (observed versus expected frequency) for rounded granite. But unlike Rocky Mountain gravel, Huangshui gravel shows a preference for angular quartzite and for rounded sandstone. In conclusion, contrasting behavior of lithologies during transport, not always apparent in raw pebble counts, is readily analyzed using contingency tables to identify the provenance of individual lithologies, including recycled clasts. Results of the analysis may help unravel river history, including changes in basin size and lithology.