Tectonic Decapitation of a Pliocene Mega-Delta on Isla Del Carmen in the Gulf of California (Mexico): And a River Ran through It

摘要

The Tiombo conglomerate on Isla del Carmen in Baja California Sur represents a mega-delta deposited mainly during the middle Pliocene (early Piacenzian age). A cross-section with the characteristically arched profile of a fan delta is exposed in sea cliffs 60 m high that extend for 2 km along the east side of the fourth-largest island in Mexico's Gulf of California. A tidewater aspect to the delta is proven by the inclusion of fossil-rich lenses dominated by the pecten Patinopecten bakeri, which also occurs abundantly together with many other middle Pliocene fossils in the Arroyo Blanco limestone juxtaposed against the south flank of the delta construction. Inland, the conglomerate fills a broad, cross-island channel approximately 3 km(2) in area that ends with cliff exposures of a lesser thickness overlooking the Carmen Passage between Isla del Carmen and the peninsular mainland of Baja California. Based on exposures that fail to reveal the full basal contact, the combined fluvial and deltaic parts of the system are conservatively estimated to exceed 200 million m(3). This study finds that the Pliocene Tiombo Delta is massively too large for the island it now occupies and that the related distributary channels in the delta's original watershed extended to the peninsular mainland and the Sierra de la Giganta in the Loreto area. The direct implication is that the Pliocene delta suffered tectonic decapitation when a reactivated halfgraben subsided and that the Carmen Passage flooded only in later Pliocene time. A useful comparison in scale is provided through examination of the modern delta at Loreto, together with its associated watershed on the peninsular mainland. Long-term changes in El Nino Southern Oscillation cycles are appraised as an influence on regional Pliocene delta construction at several other localities along the peninsular gulf coast of Baja California.