Page 206 - Carbonate Facies in Geologic History

Townsend-Kemnitz Field-A Tubiphytes and Tubular Foraminiferal Reef 193

5 N
... 3 I. • -?-
5
2
•
LOOr
200
o

Vertical
scale c-
Fig. VI-21. Typical facies changes from electrical logs from north to south several kilometers
across Kemnitz Field. For detailed petrography of darkened area see Fig. VI-22. From Ma-
lek-Aslani (1970, Fig.4), with permission of American Association of Petroleum Geologists

these along the shelf margin reaches more than 30 m and thins within 2 km north
of the margin to approximately 25 m. Farther northward, it continues thinning
for many km and disappears. These relationships can be easily ascertained from
electrical logs (Fig. VI-21). In detail the porous buildup is formed by two half
cycles separated from each other by shale, the upper, thinner unit being transgres-
sive over the lower. The basinward slope is from 2 to 4 degrees. The total length of
the recognized carbonate shelf margin is more than 50 km (Fig. VI-20).
Malek-Aslani (1970) described the chief facies developed in the buildups. The
reeffront and top is formed by a narrow belt (about 2 km wide) composed of rock
with boundstone lattice made of 30-40% Tubiphytes and tubular foraminifera.
Tubiphytes is also particularly abundant in flanking bed packstones. Its growth
habit was that of an encruster in agitated water; it apparently lived best on the
exposed flanks and tops of algal plate mounds.
The rest of the sediment in the shelf margin belt is bioclastic detritus. There is
some fine lime sediment constituting matrix. The very top beds of the shelf margin
may be composed of grainstones with rounded fragments (Fig. VI-22). Con-
structed cavities within organic framework form shelters for the fine matrix,
affording abundant geopetal fabrics. There is little spar cement and considerable
primary porosity, according to Malek-Aslani.
Shelfward of this belt lie facies considered backreef and protected from wave
action by the extended reef flat formed along the very gentle slope. These sedi-
ments consist of "backreef' grainstones with much Tubiphytes and packstones
with some matrix and less Tubiphytes. More micritic sediments contain dasyclad-
aceans and may represent shoals in the backreef lagoon. Essentially, however,
these backreef sediments represent deposition in normal marine, openly circulat-
ing waters showing that the shelf margin was at times completely inundated and
did not restrict circulation behind it. Platy algae also constitute an important part
of these sediments, appearing in more protected places behind the reef as well as
down its foreslope. They are also more abundant in lower and middle parts of the
sequence.
The foreslope facies contains an apron of packstone and wackestone. Some of
these beds are depositionally crossbedded with as much as 45° dips. The sedi-
ments are rather coarse, bimodal, bioclastic debris mixed with black, spiculitic