46 



GEOLOGY AND QUICKSILVER DEPOSITS, NEW ALMADEN DISTRICT, CALIFORNIA 



assigned the group to the Late Jurassic (Titho- 

 nian). Subsequent to the publication of this sum- 

 mary, an ammonite, Douvilleicera# sp., of unquestioned 

 Early Cretaceous (Albian) ape, was discovered in 

 San Francisco, which is generally regarded as the 

 type locality of the Franciscan group. This discovery 

 and its significance is described in an article by 

 Schlocker, Bonilla, and Imlay (1954, p. 2372-2381). 

 The information now available indicates that some 

 of the rocks of the Franciscan group were deposited 

 in Late Cretaceous (Cenomanian) time, and some 

 were deposited in Early Cretaceous (Albian) time. 

 Other beds, correlated with this group chiefly on the 

 basis of their content of greenstone and chert, fire 

 said to grade upward into, or be overlain by, Knox- 

 ville shales, and therefore they are at least as old as 

 Late Jurassic. It thus appears that we are dealing 

 with either a thick formational unit ranging in age 

 from Late Jurassic to Late Cretaceous, or that we are 

 dealing with two similar sequences, one of Tithonian 

 or older age and the other Albian and Cenomanian 

 age, and have failed to find criteria by which they 

 can be distinguished. Until this problem can be 

 solved, it seems best to designate the Franciscan rocks 

 as Late Jurassic and Cretaceous. 



THICKNESS OF THE FRANCISCAN GROUP 



The thickness of the Franciscan group cannot be 

 determined accurately in the district because of its 

 structural complexity, and because neither its base 

 nor top is present. The part of the group that is 

 present is believed from a study of cross sections to 

 be at least 10,500 feet thick, and it may be consider- 

 ably thicker. 



The minimum thickness of the part of the Fran- 

 ciscan group present in the area can be approxi- 

 mated by utilizing the partial sections exposed in the 

 various fault blocks. The older part of the group, 

 consisting of feldspathic graywacke, buff siltstone, and 

 overlying greenstone and limestone, forms a block in 

 which no major faults were found in the area between 

 Blossom Hill and the Limekiln fault. (See section 

 A-A', pi. 1.) The sedimentary rocks are about 6,000 

 feet thick, and the overlying greenstone is about 1,500 

 feet thick. On Los Capitancillos Ridge northeast of 

 the Enriquita mine another unfaulted section more 

 than 4,000 feet thick contains 3 sequences of green 

 stone separated by sedimentary rocks. A positive cor- 

 relation between these 2 partial sections cannot be 

 made, but a minimum thickness for the rocks exposed 

 in the 2 blocks can be obtained by assuming that the 

 greenstone at the top of the older block is the equiva- 

 lent of the lowest greenstone exposed in the Los Capi- 

 tancillos block. Using this assumption, we find that 



there are about 3,000 feet of additional section pres- 

 ent on Los Capitancillos Ridge, making a minimum 

 thickness of 10,500 feet for the composite of the 2 

 blocks. Some of the deep workings in the New Al- 

 maden mine, however, passed through thin greenstone 

 layers lying deeper in the section than the greenstone 

 exposed on the surface, and if this is correlated with 

 the greenstone of the older block, the composite sec- 

 tion would be 2,000 feet thicker. 



The relation of the rocks of the other blocks south 

 of the Shannon fault to these two sections are not well 

 known; but the blocks contain limestone, which can 

 probably be used as a key bed, and this suggests that 

 they contain equivalent sections. Xo limestone occurs 

 in the Franciscan group north of the Shannon fault 

 zone, and no correlation across this fault is possible 

 in the New Almaden area. 



ORIGIN OF THE FRANCISCAN GROUP 



The assemblage of rocks in the Franciscan group is 

 typical of eugeosynclinal accumulations found in oro- 

 genic belts throughout the world. The geosyncline in 

 California was at least 550 miles long and more than 

 150 miles wide. If the assemblage of more meta- 

 morphosed but otherwise lithologically similar rocks 

 found on the Palos Verdes Hills (Woodring and 

 others, 1946, p. 12-13), on Santa Catalina Island, on 

 the islands off Lower California, and in the Western 

 Cape region of Lower California (Beal, 1948, p. 36- 

 37) are included in the group, the length of the geo- 

 syncline was more than 1,000 miles. The feldspathir 

 character and angularity of the grains in the gray- 

 wacke indicate that this rock was derived from a 

 rugged, probably actively rising, landmass not far 

 distant from the site of deposition. Taliaferro (1943b. 

 p. 187-188) has concluded from the composition of the 

 pebbles in the conglomerates, and from a general 

 coarsening of the sediments westward, that the prin- 

 cipal landmass lay to the west of the depositional 

 trough. This conclusion may be justified, but the lack 

 of fragments showing myrmekitic or graphic inter 

 growths in the sedimentary rocks of the Franciscan 

 group, and the scarcity of orthoclase suggest that the 

 Santa Lucia granodiorite was not an important source, 

 either because it was not exposed by erosion or because 

 it had not yet been intruded. 



That the group accumulated rapidly is indicated by 

 the character of the sedimentary rocks, the scarcity of 

 fossils, and the great thickness of the volcanic rocks. 

 Despite this rapid accumulation, however, some pro- 

 gressive changes are shown by the differences between 

 the older and younger parts of the group. The older 

 part contains little volcanic material, chert, or con- 

 glomerate, and in general is better sorted and more 



