Growth of the Ballena fan delta on the Gulf of California (Mexico) at the close of the Pliocene Warm Period

Abstract

Tectonic uplift on the shores of Bahia San Rafael in Mexico’s upper Gulf of California exposed a Pliocene delta system that covers a map area of 4 km2. Subaerial dissection by arroyos entrenched during Pleistocene and post-Pleistocene time carved cross-sectional slices through the delta, showing its dominant construction resulting from massive transfers of siliciclastic sand derived from the breakdown of igneous rocks (tonolite) in a well-defined terrestrial basin. Restoration of the sedimentary structure by elimination of Pleistocene arroyos and linkage of former topographic lines reveals a triangular shape recognizable as a classic fan delta. The complex includes an alluvial fan that emerges from a small opening in the landscape connected to a semi-circular, high-walled basin with a map area of 2.6 km2. Through a strictly longitudinal sequence, estimates of the excavated basin’s volume and the delta’s sedimentary volume were conducted as a mass-balance exercise that yielded a strong match. The lower central part of the delta features dense concentrations of sand dollars (Dendraster granti) that form a distinct limestone coquina not previously recognized elsewhere in Baja California. Through the regional biostratigraphy of concurrent range zones supplemented by absolute age dates from inter-bedded volcanics in other places, a later Pliocene age around 3–2 Ma is suggested for the sand dollars and the delta complex in which they are buried. Such timing corresponds to the close of the Pliocene Warm Period, during which a persistent El Nino climate brought tropical storms and excessive rainfall to the upper Gulf of California. Comparisons with other Pliocene deltas throughout the Gulf of California underscore the unique status of the complex named the Ballena fan delta.