Week 6 | Geological Interpretation

San Joaquin Valley - CA

This weeks trip takes us to the San Joaquin Valley which connects Californias coast ranges to the Sierra Nevada mountain range.  The San Joaquin Valley is a sediment-filled depression known as a Basin and is classified as a forearc basin meaning it forms in front of a mountain range (Sierra Nevada Range).  The valley dates back 65 million years during the Mesozoic era when subduction was taking place off the coast of California.  Shortly after the subduction took place the valley was effected during the Tertiary period and the ancient trench known as the San Andreas fault created a zone of right-lateral strike-slip motion. 

The San Joaquin Valley bounded by the west by the Californa Coast range which represents a zone of folding and thrusting (accretionary prism) and bound to the east by the Sierra Nevada which represent the ancient volcanic arc that was also associated with the subduction zone.  The valley is also called a remnant arc trench gap.  When the valley first formed it was actually an inlet sea between two mountain ranges.  Years of erosion from the Sierras created layers of sediment dumped into the valley along with coast ranges wearing down and dumping into the valley.  Eventually, the inland sea was filled to create the continental basin it is today.

The three elements of data I believe would be most useful on our trip to the valley are a topographical map, fault activity map, and the third would be a subsidence map of the valley.

Topographical maps are a great tool to get a 3d rendering of the elevation of the area in a 2d format.  This information is particularly useful for the San Joaquin Valley as it depicts the elevation increase on either side of the valley from the Coastal range on the West to the start of the Sierras on the east.  Effectively this map would provide a useful outline of the valley as it sits between the elevated ranges.

The San Joaquin Valley is surrounded by a number of fault zones including the large San Andreas fault.  Data showing how the area around the valley with active fault lines can have an effect on the geology of the valley.  This data can also be helpful in understanding earthquakes zones and the impact on the valley.

The final piece of data would be a report on the subsidence taking place int the basin.  Current subsidence maps show a rather large area of the valley which is settling or has suddenly sunk.  The causes for subsidence can be natural events such as earthquakes, soil compaction, glacial isostatic adjustment, erosion, and sinkhole formation.  Current subsidence maps actually indicate that a large portion of the subsidence in the valley has resulted from people.  The basin has been very resource filled area for Californias Agricultural market and over the years there has been massive overuse of groundwater pumping.  As underground aquifers are drained land began to settle in the resulting cave.  Over the last several years efforts have been taken to help reduce the pressure on current aquifers and many have started to recover.

San Joaquin Valley 

Introduction to theGeology of the San Joaquin Valley. (n.d.). Retrieved from http://www.sjvgeology.org/geology/