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SDAG Monthly Meeting
Wednesday, June 14, 2017

Location: The Alamo Mexican Cafe
2543 Congress Street
San Diego, CA 92110
Tel: 619-296-1112

FROM INTERSTATE 5: Take the OLD TOWN AVENUE exit. Proceed east on Old Town Avenue. Turn left on to San Diego Avenue and then left at the fork onto Congress Street. The restaurant is located approximately three blocks to the north on the left (west) side of the street.
FROM INTERSTATE 15: Take CA 163 south to Mission valley and then take Interstate 8 west. Take the Taylor Street/Hotel Circle exit and head west on Taylor Street. Turn left (south) onto Congress Street. The restaurant is located approximately three blocks to the south on the right (west) side of the street.
happy hour
5:30pm -
Social hour  

Menu: Mexican Buffet and Cash Bar
6:30pm -

Cost: $25.00 for non-members, $20.00 for members, $15.00 for students. if pre-registered by the deadline, $5 extra if you did not make a reservation. Click the SDAG member checkbox on the reservation form if you are a member.

Reservations: Make your reservation online by clicking the button below no later than NOON, Monday, June 12. RESERVATIONS CANNOT BE ACCEPTED AFTER Monday at noon. Late reservations/cancellations are preferred over walk-ins or no-shows. Fees payable at the meeting or pre-pay with PayPal.
As a new payment option, there will be a phone credit card reader at the meeting.

If you are a current SDAG member and are not getting e-mail announcements,
make sure the SDAG secretary has your correct e-mail address.

7:30pm -

Speaker: Jorge Ledesma


Abstract: The south side of the 700,000-160,000 year-old volcanic cone on Isla Coronados (Baja California Sur, Mexico) forms a shelf that converges on older Miocene andesite from the Comondú Group. Later Pleistocene carbonates accumulated on and around the antecedent topography as related by stratigraphic sections strategically located with respect to small andesite islets that formed a fixed barrier along the outer margin of a large lagoon at the foot of the volcano. Distinct facies show the progressive foundering on the island and the infilling of the lagoon about 121,000 years ago during events correlated with marine isotope substage 5e. On the seaward side of the barrier, a basal conglomerate of andesite boulders and cobbles grades into limestone with a diverse shelly fauna and whole rhodoliths in a matrix of rhodolith sand. Similar limestone is found on the lagoon side of the barrier, but features the bivalve Pina corteziana from a sheltered environment. Other facies are represented by populations of the coral Porites panamensis at different levels of growth and integration. Biocalcarenite derived from the debris of crushed rhodoliths occurs as the most extensive facies in terms of area and thickness. Sheeted layers that dip 20° off the top of the islets toward the volcano are regarded as washover deposits typical of barrier systems. A cobble pavement, interpreted as a ravinement surface, marks a widespread unconformity at the top of the biocalcarenite. This surface was the foundation for a short-lived rocky-shore biota in transition to dense growth of branching P. panamensis. The sequence ends with a thin marine terrace deposit that buried the coral thickets at the present 12 m. level.

Below an excerpt from the volume Gulf of California Costal Ecology: Insights from the Present and Patterns from the Past. Rhodolith Banks. Some species of coralline red algae grow concentrically around a pebble or a shell fragment and remain unattached on the sea floor, adapting a free-rolling spherical or semi-spherical shape. Rhodolith (meaning "red stone") is the name applied to this sort of biological concretion. In life, rhodoliths are dark red to rose colored. Vast banks of rhodoliths accumulate throughout the Gulf of California in water as shallow as about 2 m. A good example of a rhodolith bank is found between Isla Coronados and the peninsular mainland at Punta Bajo, in this case dominated largely by a single species (Lithothamnion margaritae). Major storms sometimes sweep a multitude of rhodoliths onto the shore to be stranded in a supra-tidal setting, where they quickly bleach under the sun and expire. Individual rhodolith colonies are typically 5 cm in diameter, although the largest exceed 20 cm in diameter as shown by samples from Punta Bajo. Fossil rhodoliths commonly contribute contribute to the fabric of Pleistocene and Pliocene limestone deposits throughout the Gulf of California.Sandy Beaches and Coastal Dunes. The tides and waves that sweep clam-flats and rhodolith banks are the means by which biological carbonates are shifted landward to become a significant component of beaches. Furthermore, prevailing winter winds are capable of causing beach deflation by which the finer fraction of beach sand is blown inland to make coastal dunes. The popular camping place at El Requesón south of Mulegé in Bahía Concepción features a beach and tombolo that connects the mainland to Isla Requesón at low tide. Healthy rhodoliith banks are located off the north and south ends of Isla Requesón and the beach is dominated approximately 80% by the debris of wave-crushed rhodoliths. Beaches and related dunes can be enriched by the finely fragmented debris of clams and other mollusks, as found in Isla del Carmen.

Jorge Ledesma-Vázquez taught undergraduate and graduate courses in oceanography, and coastal sedimentology as a member of the Facultad de Ciencias Marinas (Area of Geology) in Ensenada over a 36-year career becoming an Emeritus Professor in 2014. He earned a degree in engineering geology from the Instituto Politénico Nacional (IPN) in Mexico City in 1977, a Master from San Diego State University in 1991 with the support of Dr. Patrick Abott, and a PhD from the Universidad Autónoma de Baja California in 2000. He holds the national distinction of Invesigador Nacional in Mexico, a rating associated with high performance by federal research agencies. His earliest visit to Baja California occurred in 1975 as part of a field trip with classmates from IPN, for which he obtained funding through a personal appeal to the president of Mexico. He has co-led many research expeditions and field courses involving professionals and students from SDSU, UCLA, UABC, Williams College, UNAM and many other institutions. Since 1985 he has been consulting as part of different companies such as Pelagos Co., Dames and More/Fugro, and John Minch and Associates. Workshops for Mexican agencies such as Comisión Federal de Electricidad (CFE) Geothermal branch. More recently, he has participated in field studies in the Cape Verde, Canary, and Madeira archipelagos that share some similarities with geological processes with the Gulf of California islands. Recently he has been collaborating with the Instituto Nacional de Antropología e Historía in several archeological sites in Baja, as well as with Natural Protected Areas. Such as Parque Nacional Bahía de Loreto. With several published volumes, the jewel of the crown is Gulf of California Costal Ecology: Insights from the Present and Patterns from the Past, published in 2016 by Sunbelt Publications.

Upcoming SDAG meetings - 2017

July 19: Michael Reader (Joint Meeting with South Coast and AEG) Investigations in the Hollywood AP Fault Zone

August 9 (2nd Wednesday): Timu Gallien and Ron Flick (Joint Meeting with Society of Military Engineers)

September 20: TBD

Meetings are usually scheduled for the 3rd Wednesday evening of the month. Meeting information on this website is normally updated the second week of the month.

If you have any information, announcements, ads or suggestions for an upcoming newsletter, please submit it to Ken Haase, (2017 SDAG Secretary). Any news regarding upcoming events that may be of interest to the Association or news of your business can be submitted. The submittal deadline for the next SDAG newsletter is the last Friday of the month.
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