Groffie, F.J., 1985, Geology and Late Cretaceous depositional environments in the northern corner of Camp Pendleton, southern
California: M.S. thesis, San Diego State University, San Diego, California.
Roth, J.C. 1958, Geology of a portion of the southern Santa Ana mountains, Orange County, California: M.S. thesis, University of
California, Los Angeles, Los Angeles, California.
Stevenson, R.E., 1948, The Cretaceous of the southern Santa Ana mountains, California: M.S. thesis, University of California,
Los Angeles, Los Angeles, California.
Tan, S.S., 1999, Geologic map of the San Clemente 7.5' quadrangle, Orange and San Diego Counties, a digital database:
California Division of Mines and Geology, ftp://ftp.consrv.ca.gov/pub/dmg/rgmp/Prelim_geo_pdf/ san_clemente.pdf.
Tan, S.S., 2001, Geologic map of the Margarita Peak 7.5' quadrangle, San Diego County, a digital database: California Division of
Mines and Geology, ftp://ftp.consrv.ca.gov/pub/dmg/rgmp/Prelim_geo_pdf/ margarita_pk.pdf.
Recently I was pleased to discover that the California Geological Survey had published new geologic maps of portions of southern California, including those by Siang S. Tan (1999, 2001). Unfortunately, previous work used in preparing those maps appears to include only (internal) files readily accessible to the researchers. These researchers could have benefited much from data developed by geologists from the University of Southern California, University of California at Los Angeles, Cal Tech (Pasadena), California State University, Los Angeles, and San Diego State University, if only to make their job easier.
Nevertheless, the maps by Tan (1999, 2001) are gorgeous and skillfully prepared in most respects.
Mapping by Groffie (1985)
San Clemente quad by Tan (1999)
Margarita Peak quad by Tan (2001)
The discussion below moves, geographically, in clockwise fashion. I begin at the northern corner of Camp Pendleton. This point almost corresponds with the four-
Tan (2001) versus Groffie (1985)
Locations are given by north latitude and west longitude (negative). Locations are approximate. They generally indicate the center of an area that may be some 2,000 to 5,000 feet in radius. Below, when I say “the contact,” “they map,” or something similar, I'm speaking of features shown on the 2001 map by Tan (2001) when compared to features shown on the 1985 map by Groffie.
33.497, 117.481 The KJ–Kt boundary is more sinuous.
33.489, 117.473 In the area of White Oak Springs, the KJ–Kt contact is mapped some 2,000 ft farther to the southwest. I can’t agree, because I mapped a definite contact in this area.
33.471, 117.478 They map extensive Qoa deposits extending up to elevation 1200 ft. I have difficulty agreeing.
33.462, 117.472 They map Qoa extending southeastward up this small canyon. I doubt it extends this far southwest higher than about Elevation 600 ft.
33.466, 117.466 The KJ –Kt contact is some 2,000 ft more southwesterly. This may possibly be correct.
33.454, 117.450 This highly sinuous KJ–Kt contact agrees closely with mine, in a location where I show a definite contact.
33.443, 117.453 Their mapped KJ–Kt contact is more sinuous than mine but corresponds well with mine.
33.444, 117.457 Their map lacks a fault trace. I remember this as a distinctive fault trace, with significant downdip offset, exposed in a northward-
33.431, 117.457 The contacts here correspond well with mine for a distance of about a mile north and south of this location. Their map differentiates between KJm and Kgt in this area, and thus their contact is trustworthy.
33.423, 117.482 Here, where the Kt–Kws contact crosses Jardine Canyon, the mappers send the contact eastward. On my map, the contact trends southeastward, in continuity with its trend northwest of Jardine Canyon. Thus Kws covers more ground east of Jardine Canyon on the map than it does on mine. The mappers may have spotted well-
33.408, 117.485 The map shows a Qoa deposit and Kws–Es contact essentially as I mapped them (well-
33.401, 117.489 The map shows Qoa overlying Tsa much as I did. However, the map distinguishes between five different deposits at successively higher elevations, undoubtedly differentiating multiple stream terraces.
33.410, 117.495 The map shows several deposits of Qoa overlying Tsa that I neglected to map.
33.417, 117.495 There is good correspondence of the Kws–Tsa contact between both maps for nearly a mile in either direction from here.
Tan (2001) versus Tan (1999)
The mapping by Tan (2001) covers the Margarita Peak 7½-
Trabuco Formation on the Margarita Peak quadrangle is juxtaposed against Schulz Ranch Sandstone on the San Clemente quadrangle, moving right to left (east to west).
Schulz Ranch Sandstone on the Margarita Peak quadrangle is juxtaposed against Pleasants Sandstone on the San Clemente quadrangle, moving right to left (east to west).
These jarring juxtapositions are clearly unfortunate artifacts of mapping or map preparation. Both maps cannot be correct simultaneously. I prefer the interpretations mapped on the Margarita Peak quadrangle (Tan, 2001) over those on the San Clemente quadrangle (Tan, 1999). The former are consistent with the mapping by Groffie (1985) in this area.
The Kw–Tsa contact displays good agreement between the two maps (Tan, 2001, versus Tan, 1999).
Tan (1999) versus Groffie (1985)
Locations are given by north latitude and west longitude (negative). Locations are approximate. They generally indicate the center of an area that may be some 2,000 to 5,000 feet in radius. Below, when I say “the contact,” “they map,” or something similar, I'm speaking of features shown on the 1999 map by Tan when compared to features shown on the 1985 map by Groffie.
33.447, 117.525 Discrepancies with my mapping are abundant. I mapped the lower elevations of the canyon as exposing Schulz Ranch Sandstone.
33.460, 117.537 This is an interesting, complex part of the map. Minor discrepancies with my mapping are particularly abundant. Tsi is there, more or less as I mapped it. The fault is there, although I mapped it as queried. I did not observe the fault constituting an exposed fault contact between Cretaceous and Tertiary units, but this juxtaposition per Tan (1999) may be present. Interestingly, I observed just such a fault contact several miles to the southeast, as shown in Figure 48 in the thesis text. It is possible this fault continues for several miles to the south, as I mapped it (see Figures 47 and 48 in the thesis text). Most likely what I incorrectly mapped as an isolated (fault) shear with a 19° dip to the south was merely the basal shear surface exposed at the toe of the landslide on the south side of Talega Canyon. Here, Tan was correct and I was mistaken.
33.471, 117.525 Here, they map a stream terrace dated at 680,000 years b.p.. My mapping showed a nearly isolated exposure of Pleasants Sandstone. Here I observed sedimentologic features suggesting a low-
33.492, 117.506 Here, and for several thousand feet around, discrepancies between the mapping and mine are too many to summarize. One could quibble about the stratigraphy and the exposures they present to any geologist mapping them. The outcrop patterns of Trabuco Formation, Holz Shale Member of the Ladd Formation, or Schulz Ranch Sandstone Member of the Williams Formation in this area are open to debate. The terrain is rugged. Access is poor: dirt roads are nonexistent, and the vegetation is nasty. Stratigraphic dips are low. Contacts between the units are gradational. The contacts are especially gradational and difficult to distinguinguish by this point, moving southward from Orange County. All these factors conspire to make for a difficult job for any stratigrapher and field geologist. I was guided in my mapping by work by Stevenson (1948), work by Roth (1958), who mapped the Holz Shale Member of the Ladd Formation some 100 to 200 ft thick extending southward to the northern margin of my study area (Groffie, 1985), and by exposed beds of mudstone, which is characteristic of the Holz Shale.
In general, Tan (1999, 2001) did a far superior job mapping the various levels of Quaternary terrace deposits than did I (Groffie, 19985).