The types of rocks found exposed at and around the SDSU campus include igneous intrusive, igneous extrusive and clastic sedimentary. The relationships of these rocks has been determined by geologic mapping of the rocks and publication of a geologic map. A geologic map and an explanation for the campus area are illustrated below. As we progress through these rocks and events, examples of the rocks and where they can be seen will be illustrated. There is also a brief discussion of the geological processes that were involved in formation and subsequent history of the rocks.
Geologic map and explanation for common rock units (formations) exposed at, and adjacent to, the SDSU campus. The Plistocene Lindavista Formation and the Eocene Mission Valley Formation are not shown on the above map but are discussed in the following sections.
SDSU Campus - Mesozoic Time
Evident from the rocks that are present, conditions around what is now our campus were quite different from what they are today. It is known that around the SDSU campus the oldest rocks exposed are igneous extrusive, although just outside of the campus area there are also igneous intrusive rocks. Radiometric dating of these extrusive rocks provides a numerical age of about 120 million years. This number indicates that the rocks formed during the early part of the Cretaceous Period. These extrusive rocks are volcanic basalts that have been named - in accord with standard geologic practice - the Santiago Peak Formation (show me). The intrusive rocks are primarily granitic and form part of the Peninsular Ranges Batholith. From this information we can determine that during the Cretaceous Period this area was a tectonically active. It is likely that subduction was occurring to the west, and this generated subsurface magma chambers and numerous volcanoes erupting at the surface.
SDSU Campus - Cenozoic Time
Around the SDSU campus Cenozoic time is represented primarily by four sedimentary rock units. Three of these units are considered Tertiary (Eocene Epoch) and one is considered Quaternary (Pleistocene Epoch) in age. The oldest of these rock units is called the Friars Formation. This unit consists of fine sandstone, siltstone and mudstone and is considered to have formed in shallow marine bays and lagoons (show me). Overlying the Friars Formation is the Stadium Conglomerate, a unit composed of coarse sandstone and conglomerate with pebble to cobble sized clasts (show me). The next youngest of these rock units is called the Mission Valley Formation, and it is made up of siltstone and fine grained sandstone (show me). The lithology and sparse fossils found in these three formations suggest that they were deposited in a variety of environments including floodplain, beach, lagonal, and as a deltaic complex that covered the older igneous rocks of Mesozoic age. The youngest rock unit is the Lindavista Formation (show me), which consists of sandstone and conglomerate that formed as shallow marine, wave cut terrace deposits.
Putting this information into perspective, we can visualize the SDSU area during Eocene time as being mostly low-lying land areas and shallow marine environments. The shoreline probably shifted position, and this resulted in the lateral and vertical variation in type of sediments that were deposited. Studies of the geochemistry of the sediments and from fossil evidence indicate that the climatic conditions were probably more humid and tropical than they are today. Sometime subsequent to deposition of these rocks, the SDSU area became emergent. There is little rock record, and presumably any rocks that were deposited have been removed by erosion. A relaltively flat lying surface of erosion was developed, and this surface has been uplifted in recent geologic time (perhaps the past 2 million years). Down cutting by rivers has produced a topography of flat topped mesas (San Diego Mesa, Clairemont Mesa, Kearney Mesa, etc), separated by eroded canyons and river valleys (Mission Valley, etc.).
Another way to depict the geology of a region is by using a vertical representation to indicate the relative and numerical ages of the rocks. This representation is called a stratigraphic column. An example of a stratigraphic column for the SDSU area is illustrated below.
Stratigraphic column representing the rock units at and around the SDSU campus. The oldest rocks in the area are Early Cretaceous, and include the Santiago Peak Formation (volcanic) and granites of the Peninsular Ranges batholith. Unconformably overlying these rocks are three sedimentary formations of early Tertiary (Eocene) age. Unconfomably overlying these rocks is a sedimentary formation of Quaternary (Pleistocene) age.
If you refer to the stratigraphic column above, you will note that there are two unconformities present. One is between the Santiago Peak Formation and the Friars Formation. This unconformity indicates that there are no rocks representing the uppermost part of the Cretaceous and no rocks representing the lower part of the Tertiary. This absence can be explained by processes such as erosion and non-deposition that occurred during this interval; therefore the area must have been an exposed land area above sea level. In the absence of a rock record it is difficult, if not impossible, to determine precisely what was happening at the future site of SDSUduring the interval of time represented by the unconformity (perhaps as much as 80 million years). As we will see, however, rocks exposed slightly west and north of the campus fill in a portion of this interval. By using evidence from these other local Cretaceous and Tertiary formations, we can, by inference, provide an indication of what was occurring in this area.
The second unconformity separates the Mission Valley Formation from the overlying Lindavista Formation. The amount of time represented by this unconformity is approximately 30 million years, and as with the older unconformity described above, there are rocks adjacent to campus and around the San Diego area that fill in a portion of this interval.
At the present time the SDSU campus is about 400 feet above sea level, and the rocks are currently undergoing erosion. If younger rocks were once deposited on top of the Mission Valley Formation and other rock units, they have subsequently been eroded away. Thus, the current land surface is likely to become a future unconformity in the geologic record.
