Morphology and Distribution of Pedogenic Carbonate in Quaternary Fluvial and Marine Terraces, Central Baja California Peninsula, Mexico

The east coast of the central Baja California Peninsula, near Santa Rosalia, displays prominent Quaternary fluvial and marine terraces that have variable soil horizonation with distinct pedogenic carbonate accumulation. Carbonate coating growth within the Bk and K horizons of the soil profile is used as an index of surface stabilization age. Five arroyos (El Potrero, El Morro, Santa Agueda, Purgatorio, Boleo) containing well preserved terrace and carbonate deposits were selected as sample sites within the peninsula. RTK GPS elevation data allowed mapping of the terrace sequences across the study area. Independent ages for the terraces were determined using cosmogenic Cl-36 exposure dating and luminescence dating, producing ages ranging from 15 ka to ~500 ka. Carbonate stage morphology is correlated across the mapped and dated terrace sequence, showing increasing stages of accumulation with deposition age. Accumulation of the carbonate horizon in the lower (Late Pleistocene) terraces is characterized by thin (<0.5 mm) coatings around gravelly clasts with interstitial carbonate filaments in a sandy matrix. Pervasive carbonate horizon accumulation found in the oldest terraces (~500 ka) show distinct accretion lamination that form 0.5-6 mm-thick rinds around the clasts and completely fill the interstitial matrix. Pedogenic carbonate coatings display variable micromorphologic features including silt and clay impurities, varying CaCO₃ concentrations, and unique chemical compositions that can be diagnostic of environmental conditions at the time of deposition. Thin sections were used to characterize micromorphologic features of stage KIII-KV carbonate rinds on clasts located in the oldest terraces. Elemental mapping by SEM-EDS revealed distinct deposits of amorphous silica interlayered within the carbonate accretion laminae. Stable carbonate (C and O) isotope data is consistent with carbonate accumulation derived from soil respiration and meteoric water derived from precipitation from the Pacific. Incision of the landscape recorded by the terraces suggests a variable uplift during the Pleistocene, slightly higher towards the north.