Page 192 - Carbonate Platforms Facies, Sequences, and Evolution

its
178
"::
.
within
.
I0<J'Jl
�
�
�
D
�
l%d
principal
its
growth,
WITH
lower
SOIL HORIZON
WACKESTONES
water sl:telf facies of
lithologies
part,
indicating
on side of logs is in metres.
of
BLACK PEBBLES
NERINEIDS (Ptygmatis)
that
FENESTRAL WACKESTONE
which
this
ONCOLITIC/LITHOCLAST LMST .
the upper parts of the Tojeira.
T
--
••••••
�
P.M.
CALCRETE CRUST/BLACKENED LMST.
2
3
0
basement
IN SITU MEGALODONTIDS (Pachyrismella)
member.
BEDDING
was
incorporated
A
5
7
6
8
STROMATOLITES
IRON-RICH CRUST
Ellis, R. C. L.
buildup near Rocharforte (for location, see Fig. 5). (a)
and wackestone facies (association 8) of the Montejunto
LOW-ANGLE CROSS
well­
mudstones, but it is never found above the shallow­
the Montejunto formation to
DESSICATION CRACKS
The Tojeira member overlies the ammonitic lime
To the north of the Montejunto anticline, towards
exposed
Soil-rich sequence; (b) peritidal 'loferitic' sequence. Scale
angular
developed thrombolite boundstone is also exposed
nearby during this time. Basement clasts, along with
the northeast. Limestone breccias and shales are the
within the breccias, and become more common in
pink feldspars and other basement fragments during
contemporaneously-karstified limestones also occur
Fig. 6. Sample logs of sequences within the lime mudstone
the
tain
larly
beds
Both
which
The
The
occurs.
ditions
shoals,
massive
of
majority
nerineid
a
the
emergent.
Wilson and R. R.
mudstones
bindstones.
are
Montejunto
stromatolite
of
shelf
considerable
reef
aggradational
coral
shelf
coral
nerineid
suggests
foraminifera
(Fig.
Close
fossiliferous,
marked
wackestone
5)
interior
Leinfelder
heads
exhibiting
gest deposition below
that
breccia.
break
and
to
b)
inferred shelf-edge,
Tubiphytes-wackestones
is
a
limestones
carbonate
lithological
and
and
biostrome
and
the
facies
the
The
the
wackestones
posited on a low-angle slope.
shelf
are
with corals to form small patch�
which
and soil horizons.
containing
the
largely
normal
omission
stromatoporoids and Solenopora.
contain
area
pass
variation.
were
very shallow, restricted environment:
phaceloid
composed
dasycladaceans
few
formed
heads
was
shelf-edge
ammonitic
dominance
(association
marine
of
reef breccias deposited as storm deposits.
surfaces
normal wave-base,
of
boundary
of
of
composed
coral
abruptly
(sensu Read,
6).
lime
within this facies, and its basinward location,
Nerineids
probably
of
biota
·
mud­
de­
Thin, discontinuous sequences of oolitic and bio­
north­
crudely­
(largely nerineids when present) though some con­
sug­
wards into lime mudstones and wackestones (associ­
(Fig. 5). Grainstones also occur farther to the north­
east, where, such as at Rochaforte, they are particu­
A number of small,
developed lofer-like cycles (Fig. 6). Supratidal con­
The fenestral lime­
are
indicating. a
well-developed coral framestone knolls occurs within
limestones contain a wide range of fossils and show
1982).
concentrated in some horizons, as are heads of the
grainstone
the
abundant
stones pass into a thick sequence of wackestones and
reef framestones and
m�gafossils
the wackestones and are associated with redeposited
corals,
the
and rare
with the
clastic grainstones are taken to mark the shelf-break
micrite
These
bushes
The Montejunto buildup probably formed as an
ation 8). Though there are some leveis within the
facies associations 6 and 8 trend NW-SE, which is
break is marked by relatively minor thinning of the
tectonic evolution of the Lusitanian Basin. The shelf
parallel to an important secondary element in the
stones are associated with a thick sequence of black
pebble soil horizons exposed at Rochaforte (Fig. 6),
chaetitid Ptychochaetetes, which in places combine
frequently
Fenestral wackestones occur in the upper part of
the mudstones around the northwestern margin of
between