ASSESSING QUATERNARY TECTONIC UPLIFT IN THE ARROYO INFIERNO, SANTA ROSALIA BASIN, MEXICO, USING FIELD MAPPING AND MORPHOMETRIC INDICES

The Santa Rosalía basin is located on the eastern shore of the Baja California peninsula in México. Strike-slip faulting and extensional rifting caused basin subsidence and offset during the Late Miocene through Pliocene epochs. During the Pleistocene, volcanic activity in the region included buildup and collapse of La Reforma caldera. After eruption of extensive ignimbrites from 1.4–1.1 Ma (Schmitt et al., 2006), fluvial and marine terraces formed along the arroyos and on the coast. The main research objective of this project is to quantify neotectonic activity in Arroyo Infierno by measuring terrace characteristics such as stability of the surfaces, degree of soil development, and landscape position. In the field, strath and fill terraces were digitally mapped using QGIS on field tablets, and older and modern channel stream long profiles were plotted to identify changes in elevation with distance upstream. Soil properties were described for both the fluvial and marine terraces. In the lab, soil grain size analyses were combined with the field properties to develop a chronosequence of the terrace surfaces. A set of three distinct terraces, separated by 30–50 m of elevation gain, was observed. Younger terrace soils have a higher sand to clay content, the latter increasing in older, higher terrace soils. Bk soil horizons are thicker on the higher elevation fluvial terraces, confirming that they are older. An area of higher than average stream slope gradients was detected in upstream tributaries of Arroyo Infierno. A harder, more resistant volcanic rock type upstream of this area is the most probable explanation for this observation, rather than a migrating knickpoint. It is concluded that Arroyo Infierno has a history of relative uplift and base level change that has caused not only vertical incision and terrace preservation, but also a stream capture that displaced the location from the ~125 ka lower Arroyo Infierno about 1 km north to the present-day canyon.