SDAG Field Trip 2002

La Bufadora and the Agua Blanca Fault

After a continental breakfast of camp coffee (Starbucks!), orange juice and bagels, we headed up and over the hill to La Bufadora to meet Jorge Ledesma. He gave an impromptu lecture on the marine terraces, the Agua Blanca Fault, and the Alisitos Formation, as well as the Santiago Peak Volcanics in Baja California Norte.

Camry Field vehicle

One of the most visited places in Ensenada is La Bufadora [The blowhole], which forms a natural phenomenon unique in the zone. It consists of a fracture in rocks from the Alisitos Formation at sea level from which the waves enter. The seawater then rises and occupies the fracture. The water is the expelled by the air from the bottom of the fracture on the new wave arrival, expelling the water like a giant geyser to heights up to 15 meters.

Ensenada Area Geology
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Traveling from Ensenada to the south, the highway is on Pleistocene marine terraces [5e] that lap against metavoicanic hills to the west. Those hills are made up of pyroclastic rhyolite and dacite in a large syncline which plunges to the west. The whole section is almost 2,400 meters thick, and it constitutes the lower most part of the Alisitos Formation, the section is more lithologically related to the Santiago Peak volcanic unit, than the Alisitos Fm south of the Aqua Blanca fault. According to Wetmore and others [2002], the Santiago Peak arc segment was develop on oceanic basement that had been structurally juxtaposed with the continental margin prior to arc magmatism, and the Alisitos arc segment and its oceanic basement was exotic to North America prior to its accretion in the late Early Cretaceous. The same authors state that the currently active Aqua Blanca fault is an inherited structure that originated as a sinistral transpressional continuation of the suture that juxtaposes the Alisitos arc segment with the continent.

The Aqua Blanca Fault [ABF] is the most dominant feature in northern Baja California, the fault is geomorphically very well expressed, with strike slip features such as scarps, lateral offsetting of streams, fault controlled valleys, and hot springs. Allen and others [1960] had determined a 22 Km right lateral displacement west of Valle de Aqua Blanca.

View From La Bufadora to Southeast

The total extent of the ABF on land is 130 Km, and Rockwell and others [1993] had estimated the slip rate from late Pleistocene to Holocene to be about 4.1 to 4.3 mm/year. The fault shows three major right steeping segments that extent from the western part of Valle de La Trinidad to Valle Aqua Blanca, from Aqua Blanca to Valle de Santo Tomas, and from Valle de Santo Tomas to Punta Banda. At Punta Banda, the fault consists of a single, mostly vertical main trace with minor branches. Both the valley of Maneadero and the associated Estero de Punta Banda are a couple of geomorphic expressions developed by the activity on the fault. Punta Banda is the geographic name for the peninsula that protrudes from land into the Pacific Ocean, as a result of activity on the same ABF. It is a range up to 1,000 meters above sea level. Here, the ABF juxtaposes rocks from the Alisitos Formation, and sedimentary units from the Rosario Formation, characterized by sandstone and lesser amount of conglomerate. Most of the peninsula is made up from Alisitos material.

Rockwell and others [1989] described a suite of 14 Pleistocene marine terraces on the peninsula of Punta Banda, from 15 m for the lowermost to 300 m for the highest one. They used corals to date these terraces, obtaining an age of 80 Ka [5a] at the first terrace. Then they got 120 Ka [5e] for the third terrace. The uplift rate for late Quaternary for the peninsula is of 0.16 to 0.29 m/Ka. Based on an assumed constant rate, Rockwell and others, estimated that the fourth and fifth terraces to represent isotope stages 7 [200 Ka] and 9 [350 Ka]. Overall, the lateral displacement is the main component of slip along the ABF.

Sunday Route Map
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