FRANCISCAN GROUP 



13 



The clastic sedimentary rocks are characterized by 

 a high content of feldspar which is mostly sodic 

 plagioclase, and except in the conglomerate, by a 

 marked angularity of grains; in many there is an 

 uninterrupted gradation in grain size from the coarsest 

 clasts to the finest matrix material. Rocks with the 

 grain size of sandstone predominate, siltstone is com- 

 mon, shale less so, and conglomerate is rare in the 

 district. The rocks are poorly sorted; the sandstones 

 are true graywackes and the siltstones and shales 

 contain a large proportion of minute mineral grains 

 rather than clay minerals. In spite of the variety of 

 clastic rocks in the sedimentary pile, which is more 

 than ten thousand feet thick, they are so distributed 

 that it is impossible to divide the sequence into mappa- 

 ble units consisting wholly, or even largely, of either 

 coarse or fine sediments. Within sequences that are 

 chiefly graywacke are local sections several hundred 

 feet thick consisting largely of siltstone and shale, but 

 attempts to map these failed because the shale is not 

 persistent along the strike. Conglomerate lenses inter- 

 bedded with the graywacke are also too thin and too 

 limited in extent to provide mappable units. 



The physical characteristics of the clastic sedimen- 

 tary rocks on microscopic, hand specimen, and outcrop 

 scale indicate that they were deposited rapidly, in part 

 at least, by turbidity currents. Such poorly sorted 

 sedimentary rocks, together with the intercalated 

 chert, minor limestone, and soda-rich volcanic rocks, 

 make up an assemblage found in many parts of the 

 world. They are regarded as typical of the accumula- 

 tions that build up in a structural trough along the 

 margin of a continent when rapid deformation accom- 

 panies deposition, and are generally referred to as 

 eugeosynclinal deposits. 



GRAYWACKE 



Graywacke, which in most reports is referred to as 

 Franciscan sandstone, makes up more than half of 

 the Franciscan group. Nearly all varieties are dis- 

 tinguished easily from the other rocks in the district 

 even though the graywacke exhibits much greater 

 variation in color, in mineral content, and in general 

 appearance than do most sedimentary units. All 

 varieties are dark-colored poorly sorted dirty rocks 

 containing abundant grains of feldspar and some 

 rock fragments. (See fig. 3.) The variations they 

 show are partly the result of original sedimentation 

 and partly the result of late processes leading to dif- 

 fering degrees of induration, metamorphism, and 

 deformation. 



Several thick sequences of graywacke separated by 

 volcanic rocks are found in the district, and it was 

 at first thought that these might be mapped as sepa- 



FIGCBE 3. Massive graywacke of the Franciscan group ; actual 

 size. To bring out the texture the specimen was polished and 

 etched with hydrofluoric acid, which makes this dark-gray rock 

 appear much lighter than it really is. Irregular white patches and 

 veinlets are calcite. An analysis of this rock is given in column 2, 

 table 1. 



rate units or formations. An elaborate scheme of field 

 classification based on proportions of feldspar to 

 quartz, grains to matrix, light-colored grains to dark- 

 rock fragments, and other criteria was attempted; 

 but attempts to develop cartographic units by this 

 means failed because the differences between sequences 

 were no greater than the variations within a single 

 sequence. In general, however, the older rocks oc- 

 curring in the southern part of the district were found 

 to be more feldspathic, whereas the younger rocks 

 contain more lithic fragments. 



The exposures of graywacke are generally less than 

 a hundred feet across, but locally, as on Mount Umun- 

 hum, exposures are fairly continuous over half a 

 square mile. In some large areas, notably in the 

 Santa Teresa Hills, outcrops of these rocks are almost 

 nonexistent. The average individual outcrop does not 

 exceed 10 feet in length and consists of exceptionally 

 massive rock cut by several sets of joints. In many 

 such isolated knobs of graywacke bedding can be 

 distinguished only with difficulty, although in places 

 the alinement of mica flakes or shale fragments in- 

 dicates the attitude of an otherwise massive rock. The 

 true character of the sequence is better seen in arti- 

 ficial exposures, as in roadcuts or mine workings. In 

 these exposures the graywacke generally shows more 

 bedding and can be seen to be somewhat folded and 

 cut by faults, as shown on figure 4. Where inter- 

 layered with shale it may also show individual beds 



