Geology Reference
In-Depth Information
Plate 136 Microfacies of a Late Jurassic to Early Cretaceous Ramp: Subsurface of Bavaria (Germany)
The plate exhibits the microfacies of ramp carbonates in the Kinsau well near Augsburg in southern Bavaria. Fig.
1 on Pl. 136 is the lowermost, Fig. 10 the topmost sample. The sequence starts with Tithonian shallow-marine,
sometimes lagoonal carbonates (-> 1, 3-5), and coral reef limestones (-> 2; maximum size of the patch reefs in
outcrops 20 m), overlain by ramp deposits representing the Early Cretaceous
'Purbeck facies'
(Berriasian; ->
6-10). This facies known from England, France and Switzerland, occurs in southern Germany and northern
Austria in the subsurface. Purbeck sediments formed in strongly varying environments, and comprise marine,
brackish, freshwater and hypersaline sediments (see Fig. 4.26 for an example).
Other microphotographs can be found on Pl. 16/7 (tidal intraclasts), Pl. 29/3 (fabric-selective oomoldic po-
rosity), Pl. 65/5, 6 (charophycean brackish-water algae), and Pl. 93/5 (
Favreina
-peloid sand).
The
shallowing-upward sequence
consists of outer, mid- and inner ramp deposits (Fig. 15.16). Outer ramp
sediments, deposited below the normal storm wave base, comprise mud-rich subtidal carbonates and marls (->
1), small patch reefs (-> 2), and shallow subtidal limestones with red algae (-> 3 and 5). The existence of mid-
ramp sediments is indicated by Larger foraminifera (compare Pl. 69/1) and micrite oncoids (-> 6). Low-energy
inner ramp deposits are indicated by oncoid grainstones with porostromate cyanobacteria (-> 4). High-energy
inner-ramp sediments are represented by sand shoals and sheets (-> 7-9) controlled by tidal currents. For a
coastal sabkha see ->10. Porosity and reservoir qualities are controlled by primary depositional facies (geom-
etry and grain composition of high-energy shoals; micritic seals) and facies-selective diagenesis (freshwater
dissolution of ooids, repeated dolomitization).
1
Deeper outer ramp
(A in Fig. 15.16): Fine-grained bioclastic packstone. Grains are echinoderms (white), red algal clasts
(black) and coated shell fragments. The packstone is intercalated within mud- and wackestones, pointing to downslope
sediment transport.
2
Patch reef
(B in Fig. 15.16): Framestone consisting of intergrown corals (C), chaetetid coralline sponges (CH) and fora-
miniferal crusts (F). Arrows point to boring. Outer ramp. Ramp microfacies RMF 12 (see Fig. 14.30 for definitions).
3
Deeper outer ramp
(C in Fig. 15.16):
Red algal packstone
composed of rhodoids formed by
Marinella
Pfender. This alga
has millimeter-sized thalli and extremely fine threads (< 5 to 12
m). The alga is known from the Late Jurassic to the
Oligocene, and is common in Jurassic and Early Cretaceous platform and reef carbonates (Leinfelder and Werner 1993).
At smaller magnifications, the micritic thalli can be mistaken for micritic clasts. Note intergranular dissolution (white)
due to strong compaction.
4
Inner ramp
(D in Fig. 15.16):
Oncoid rudstone.
Larger grains are filamentous cyanobacteria (C) and gastropods (G),
small grains are algal peloids. Blocky cements and infill of intergranular voids with crystal silt (arrow) indicate meteoric
overprint. Restricted intertidal environment. RMF 21.
5
Outer ramp
(C in Fig. 15.16):
Rhodoid packstone
with
Marinella
(M). The facies interfingers with facies D. Low-energy
subtidal environment. RMF 9.
6
Protected lagoonal environment
(E in Fig. 15.16):
Oncoid wackestone
with micrite oncoids and large lituolinid foramin-
ifera (
Pseudocyclammina,
P), indicating a mid-ramp position. The micritic matrix is selectively dolomitized. RMF 13.
7
Inner ramp
(F in Fig. 15.16):
Ooid grainstone
with simple and compound, tangential ooids. Intergranular pores are
occluded by calcite cement, some of which exhibits gravitative textures indicating vadose meteoric diagenesis. High
energy inter- and subtidal shoals. RMF 29.
8
Intertidal shoals on the inner ramp
(G in Fig. 15.16): Fine-grained peloid grainstone. Most grains are fecal pellets as
indicated by their rodlike shape.
9
High-energetic shoals of the inner ramp
(H in Fig. 15.16):
Favreina
grainstone consisting predominantly of fecal pellets
of decapod crustaceans (see Sect. 10.2.4.7). RMF 27.
10
Back-ramp sabkha
(I in Fig. 15.16): Evaporitic argillaceous mudstone composed of alternating mudstone and layers with
anhydrite (white). Supratidal environment. Standard Microfacies Type SMF 25.
Fig. 15.16.
Generalized facies distribution of a Late Jurassic/Early Cretaceous carbonate ramp
based on data from the sub-
surface of southern Bavaria, Germany.
