Poway Conglomerate

Poway Conglomerate

This is a mystery story.

Now, I’m not a geologist.  But I recently discovered this great mystery story while trying to understand the soil in my yard.  My wife has been very frustrated trying to get anything to grow in this horrible dirt.  It’s sandy but it’s glued together like cement.  The best gardening tool for our yard is a crowbar.  This sand is filled different-sized stone cobbles of many colors - from grey to pink.  The stones are round like you would find on a river bottom.  But we are hundreds of feet above the mesas of San Diego.  After some research, we discovered that it’s known as the Poway Conglomerate and it has a fascinating history.  But first a little history…

About 95 million years ago, a heavy, dense oceanic plate was moving east and subducting under the North America plate. This caused great heat within the earth, volcanism on the surface, and the formation of plutons deep within the rising peninsular mountain range.  At one time, this range may have been as mighty as the Andes today.  During the late Cretaceous and Early Eocene periods, these mountains eroded and formed layer after layer of a coastal plain extending west.  Cretaceous strata include the Lusardi Formation, the Point Loma formation, and Cabrillo formation.  Eocene strata of the La Jolla group are made up of sandstones and mudstones that can be seen along Torrey Pines State Beach. 

About 45 million years ago, the Poway Group of sedimentary rocks was deposited on the coastal plain.  This group is characterized by sand mixed with pink and grey rhyolitic gravel and cobbles.  It was very different from the underlying sandstones eroded from the Peninsular Range.  The Poway Group was hypothesized to be a huge alluvial fan of a great river which carried these cobbles hundreds of miles.  As early as the late 1800’s this strata caused great excitement among geologists because THERE WAS NO SOURCE OF THESE ROCKS ANYWHERE NEAR SAN DIEGO.

I remember in the early 1980’s discussing local geology with my buddy Greg who IS a geologist.  He told me about the Poway Conglomerate and how no one had found its source.  They had found evidence of the ancient Bellena River, first described by H.W. Fairbanks in 1892, cutting west-southwest from Santa Ysabel to the San Vicente Reservoir.  Enormous deposits of Eocene river gravel can found along this ancient river valley.  That means the Bellena River came over the eroding Pennisular Range from the east.  But, again, there is no Rhyolitic source east of San Diego.  So where were the headwaters of the Bellena River?

The answer lays in the fact that at the time of the Bellena River, the Gulf of California hadn’t formed yet and San Diego was adjacent to Sonora, Mexico.  About 5.5 million years ago, after the Farallon Plate had disappeared under the continent, a large chunk of the North American plate, including most of Southern California, broke off and was dragged north by the oceanic plate.  The San Andreas fault is the boundary between this rogue piece of North America and the North American plate. California’s notorious earthquakes attest to the bumpy ride north.  In the last 5.5 million years, Baja California-San Diego-Los Angeles has moved 2.5 degrees of latitude to the north.  But this is recent history.  Long before this, the ancient Bellena River could have carried rhyolitic rocks from eroding volcanoes in Sonora, Mexico over the Peninsular Range and deposited them in San Diego.  Then San Diego and the Poway Conglomerate were carried north.  In 1989, Smith and Abbott set out to test this hypothesis.  They compared 16 trace elements in Poway rhyolite gravels and Sonoran bedrock and found strong evidence that they were one and the same. The mystery was solved.

Here is an animation which summarizes of the story of the Poway Conglomerate. It was drawn and animated by Dr. Tanya Atwater, UCSB based on Pat Abbott, The Rise and Fall of San Diego, 1999.

Question of the Day:  In the above video, a small piece of the original delta ends up in the LA area.  Where is can it be found?