Pliocene–Pleistocene rocky shorelines trace coastal development of Bahı́a Concepción, gulf coast of Baja California Sur (Mexico)

Pliocene rocky-shore deposits on the east coast of Baja California Sur (Mexico) formed under the influence of tectonic events and changing relative sea level in the Gulf of California. Depositional patterns are represented by two distinct styles of marine transgression. Level-bedded deposits in semi-enclosed basins form steep buttress unconformities similar in profile to basin fill detected through seismic stratigraphy. Such deposits are typical of sheltered rocky shores, where erosion of terrestrial materials is limited by low wave energy and/or slow rates of changing sea level. Ramp deposits form low-angle (5–9°) unconformities on exposed rocky shores, where erosion is enhanced by high wave energy and/or rapid rates of changing sea level. These conform to the pattern of coastal or marine onlap in seismic stratigraphy. With an area of 270 km2, Bahı́a Concepción is one of the largest fault-bound bays in the Gulf of California. It occupies a half-graben structure that opens on the gulf to the north. An additional 100 km of coastline was formed after Pliocene flooding of the Bahı́a Concepción graben. Sheltered rocky shores are represented by buttress unconformities between Miocene volcanics (Comondú Group) and Upper Pliocene strata in small, connected basins at the southeast end of the bay. Dating is based on Clypeaster marquerensis (sand dollar) and associated mollusks in limestones from the Bahı́a Concepción Member of the Infierno Formation. Other unconformities with the Comondú on the open gulf coast adjacent to Bahı́a Concepción take the form of carbonate ramps. At Punta Chivato, 20 km north of the bay mouth, the San Marcos Formation is traditionally considered Early Pliocene in age based on Clylpeaster bowersi (echinoid) and Solenastrea fairbanksi (coral). At San Nicolas, 15 km east from the southern terminus of the bay, C. bowersi likewise dates the basal Pliocene. Parallel to the axis of the bay, the Bahı́a Concepción fault zone is no older than the Miocene, because it cuts through the Comondú Group. The main expansion zone for the protogulf jumped over this zone to the east, where Lower Pliocene conglomerates were deposited on a faulted coast between Punta Chivato and San Nicolas. Along this front, eroded andesite cobbles predominate as a component of the basal Pliocene, but rare granodiorite cobbles indicate a Cretaceous source brought to the surface from considerable depth. Punta Chivato, Bahı́a Concepción and the San Nicolas areas retain widespread Upper Pleistocene rocky-shore deposits unconformable with Pliocene strata or the Miocene Comondú. Granodiorites eroded from Punta San Antonio also form the basis for an Upper Pleistocene rocky-shore. These deposits usually are near the 12-m elevation regionally associated with substage 5e terraces (120–135 Ka). Their uniformity in elevation inside and outside Bahı́a Concepción indicates that minor tectonic uplift was locked in step throughout the immediate region by that time.