COAL  IN  THE  MOUNT  DIABLO  RANGE,  MONTEREY 
COUNTY,  CAL. 
By  Ralph  Arnold. 
Location  and  topography. — A  deposit  of  coal  a  of  commercial  importance  and  also  worthy 
if  note  because  of  its  occurrence  in  such  a  comparatively  recent  formation  as  the  Lower 
Jiocene  is  found  in  Stone  Canyon,  near  the  middle  of  the  Mount  Diablo  Range,  Monterey 
Jounty,  Cal.  The  range  in  this  region  consists  of  a  main  divide,  attaining  an  elevation  of 
»ver  o,000  feet,  and  several  less  prominent  parallel  ridges,  all  trending  in  a  general  southeast- 
lorthwest  direction.  The  coal  outcrops  about  midway  down  the  western  flank  of  the 
nain  ridge.  The  locality  is  about  23  miles  northeast  of  Bradley,  and  may  be  reached  over 
agon  road  from  either  Bradley,  San  Miguel,  or  Coalinga. 
Geology  and  structure. — The  formations  involved  in  the  geology  of  the  range  in  the  imme- 
liate  vicinity  of  the  coal  deposit  are  the  serpentine,  chert,  and  other  metamorphosed  rocks 
if  the  Franciscan  (pre-Cretaceous)  complex,  the  Lower  Miocene  sandstone,  and  the  basal 
andstone  and  white  shale  of  the  Santa  Margarita  (Upper  Miocene)  formation.  The  Fran- 
iscan  forms  the  core  of  the  main  ridge,  outcropping  intermittently  along  its  top  and  also 
n  a  band  along  its  western  flank.  The  Lower  Miocene  sandstone,  at  the  base  of  which  is 
he  coal  bed,  overlies  the  Franciscan  unconformably  and  forms  a  steeply  northeastward- 
lipping  belt  along  the  eastern  side  of  the  western  band  of  Franciscan.  A  thickness  of 
,t  least  500  feet  of  the  Lower  Miocene  sandstone  is  exposed  in  the  vicinity  of  Stone  Canyon. 
U)out  35  feet  above  the  top  of  the  coal  bed  is  a  thin  fossiliferous  stratum  in  the  sandstone, 
rielding  the  fauna  which  determines  the  age  of  the  formation.  The  Santa  Margarita  beds, 
onsisting  of  from  100  to  200  feet  of  coarse  sandstone  overlain  by  1,000+  feet  of  thinly 
animated  hard  white  shale,  rest  unconformably  upon  the  Lower  Miocene  and  cover  con- 
idcrable  portions  of  the  main  ridge.  In  the  immediate  vicinity  of  Stone  Canyon  the  Santa 
Margarita  forms  a  southeast-northwest  syncline,  which  separates  the  Franciscan  area  on 
he  top  and  northeastern  flank  of  the  main  divide  from  the  Franciscan  and  coal-bearing 
jower  Miocene  outcrops  on  the  southwestern  flank. 
The  local  structure,  as  shown  by  fig.  9,  is  essentially  that  of  a  syncline  of  the  Santa  Mar- 
garita sandstone  and  shale  resting  on  a  post-Monterey  (Middle  Miocene)  erosion  surface. 
"Tie  Lower  Miocene  formation,  including  the  coal  at  its  base,  is  separated  from  the  Fran- 
iscan  complex  by  an  unconformity,  and  possibly  also  by  a  zone  of  faulting. 
Coalbed. — As  previously  stated,  the  productive  coal  bed  occurs  at  the  base  of  and  con- 
ormable  with  the  lower  Miocene  sandstone.  The  bed  averages  between  14  and  16  feet  in 
hickness,  and,  in  the  places  where  it  has  been  exploited,  has  a  dip  of  about  70°  N.  40°  E. 
it  some  places  along  the  outcrop  where  apparently  the  creep  of  the  rocks  down  the  side 
iill  has  affected  it,  the  dip  is  as  low  as  30°  and  the  coal  bed  is  pinched  out  to  an  exposure  less 
,a  A  brief  description  of  this  deposit  and  of  the  Stone  Canyon  coal  mine  is  given  in  the  Eighth  Ann. 
?ept.  Cal.  State  Mineralogist,  1888,  pp.  403-404. 
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