1978 



I.IMFSrONF IN Till COAST RaNCSFS 



39 



Limestone with chert interbeds 

 Franciscan Fm. 



Inclined Vertical Overturned 



Strike and dip of beds 



Figure 3. Map of Permanente and Monte Belle Ridge limestone deposits 



The limestone on Monte Bello Ridge occurs as len- 

 ticular, discontinuous masses in a thick sequence of 

 volcanic and sedimentary strata of the Franciscan For- 

 mation. The strata dip predominantly northeast but 

 in man\ places are vertical or overturned to the south- 

 west (bibblee, iy66b). Evidence of faulting is preva- 

 lent throughout the area, and some minor faults offset 

 some of the limestone lenses. The limestone every- 

 where is associated with thin chert interbeds. Most 

 lenses of the limestone and chert are less than 50 feet 

 thick and seldom as thick as 100 feet. However, the 

 lens in NE'/i sec. 29 is reported to be as much as 200 

 feet thick and to dip vertically (Dibblee, 196.'i, oral 

 communication). The limestone is typical of the Fran- 

 ciscan Formation, varying from light gray, fine 

 grained, and foraminiferal to dark gray, fine to me- 



dium crystalline, and bituminous. Silica in chert beds, 

 as well as distributed interstitially with the calcite, 

 constitutes the principal impurity. The maximum 

 grade of selected limestone samples is indicated by 

 analyses in Franke (l^.^O, p. <>, H)) and Huguenin and 

 Costello (1^20, p. 1«5). Four analyses (made by Abbot 

 A. Hanks, Inc., in 19.';6) of selected samples from the 

 Burns Ranch in sec. 15 — containing 51.1-54.1% CaO, 

 0.2-0.5% MgO, 0.8-4.5% Si02, and small percentages 

 of FeoOj, Al.Oi, and P;(),— are rather typical of the 

 Monte Bello Ridge limestone deposits. 



Reserves of limestone no doubt aggregate many mil- 

 lions of tons, but none of the deposits appears to be 

 very large. Furthermore, many of them dip steeply 

 into the ridge, thereby limiting the available reserves. 

 The apparent small size and discontinuous nature of 



