If we travel approximately 20 miles west of campus and up the coast - north of Mission Beach - to Tourmaline Beach we find rocks exposed along the sea cliffs. Because of the strike and dip of these rocks a traverse northward along the base of the cliffs provides exposures of progressively older and older rocks. The oldest rocks are found at the base of the cliffs are at False Point. Although these rocks are all sedimentary, they have different lithologies, have different names, and represent somewhat different time intervals than those around the campus area. Using similar techniques of geologic mapping the relationships of these rocks are known. An important task for geologists is to determine how the rocks from these two areas are related. This technique is known as correlation (see Lab 6, Geologic Time)
Geologic map and explanation for the Tourmaline Beach area. Some of the rocks discussed below can not be seen on the map because the cliffs along the coastal zone are nearly vertical. Hence, the explanation for the above map does not include all of the units discussed below.
Tourmaline Beach - Meosozic Time
The oldest rocks exposed along the cliffs at Tourmaline Beach are to the north end of the beach at False Point. These are sedimentary rocks that are mostly conglomerate and some very coarse grained sandstone called the Cabrillo Formation (show me). These rocks were probably deposited in shallow marine to beach conditions. Although few fossils have been found in the Cabrillo, fossils in associated formations provide evidence of a Cretaceous age, but somewhat younger (perhaps 40 million years younger) than that of the Santiago Peak Formation exposed at SDSU.
Tourmaline Beach - Cenozoic Time
Cenozoic rocks exposed at Tourmaline Beach are sedimentary rocks that include conglomerate, sandstone, and mudrock. The oldest of these are pebble to cobble conglomerate and sandstone of the Mt. Soledad Formation (show me). These rocks were deposited as channel fill in beach environments. Conformably above the Mt. Soledad Formation are mudstone and siltstone of the Scripps Formation ) (show me). These rocks were deposited in environments ranging from tidal flat to submarine fan (show me). Fossils in these and laterally equivalent formations in other local areas indicate that both formations are of Early Cenozoic age (Eocene), but are slightly older than those at SDSU.
Overlying these Eocene rocks are sedimentary rocks consisting of conglomerate and fine grained sandstone called the San Diego Formation (show me). The conglomerate has cobble sized clasts and occurs at the base of the formation. The remainder of the formation consists of sandstone. These rocks are not well cemented and tend to crumble easily. They were formed in intertidal and beach environments. Fossils are abundant in some layers of the formation, and indicate an age of Quaternary (Pliocene).
The youngest rocks at Tourmaline Beach are terrace deposits that are probably of Pleistocene age. They consist of mud, sand, pebbles and cobbles that are weakly cemented together and tend to erode easily. Not a great place to build a house! The stratigraphic relationships of these rocks are illustrated below.
Stratigraphic column representing the rocks exposed along the cliffs at Tourmaline Beach. The oldest rock unit is the Cabrillo Formation of Early Cretaceous age. Unconformably overlying the Cabrillo are two Eocene Formations of early Tertiary (Eocene) age. Unconformably overlying these units is the San Diego Formation of Tertiary (Pliocene) age, which is, in turn overlain by poorly consolidated terrace deposits of Quaternary (Pleistocene) age.
Some of the sedimentary formations at Tourmaline Beach are similar to those at SDSU in that they reflect marginal marine beach environments to shallow marine conditions. However, some of the rocks, especially parts of the Cabrillo and San Diego Formations, have been interpreted as representing open to possibly deep marine environments. There are no rocks around the campus that reflect these environments.
As with the SDSU area, there are unconformities at the Tourmaline Beach section. One occurs between the Cabrillo Formation and the Mt. Soledad Formation. Rocks missing represent the uppert part of the Cretaceous and the lowest part of the Tertiary rock record. The other unconformity occurs between the Scripps Formation and the San Diego Formation (show me), where a portion of the upper part of the Tertiary rock record is missing.
