Geology Reference
In-Depth Information
Flügel, E., Kiessling, W. (2002): A new look at ancient reefs.
- In: Kiessling, W., Flügel, E., Golonka, J. (eds., 2002):
Phanerozoic reef patterns. - SEPM, Special Publications,
72
, 3-10
Haywick, D.W., Kopaska-Merkel, D.C. (eds., 2001): Car-
bonate mounds: sedimentation, organismal response, and
diagenesis. - Sedimentary Geology,
145
, Issue 3-4, 157-
414
Hubbard, D.K., Burke, R.B., Gill, I.P. (1998): Where is the
reef: the role of framework in the Holocene. - Carbon-
ates and Evaporites,
13
, 3-9
Hüssner, H.M. (1994): Reefs, an elementary principle with
many complex realizations. - Beringeria,
11
, 3-99
Insalaco, E. (1998): The descriptive nomenclature and clas-
sification of growth fabrics in fossil scleractinian reefs. -
Sedimentary Geology,
118
, 159-186
James, N.P., Bourque, P.-A. (1992): Reefs and mounds. -
In: Walker, R.G., James, N.P. (eds.): Facies models. Re-
sponse to sea level change. - 323-348, Ottawa (Geologi-
cal Association of Canada)
Kiessling, W. (2002): Secular variations in the Phanerozoic
reef systems. - In: Kiessling, W., Flügel, E., Golonka, J.
(eds.): Phanerozoic reef patterns. - SEPM, Special Pub-
lication,
75
, 625-690
Plate 145 Ecologic Reef or Something Else? The Capitan Reef, (Guadalupe Mountains), Texas and New
Mexico
The origin of the reef rocks and the ecologic interpretation of the organisms of the famous Capitan reef have
produced a long ongoing debate. The Late Permian Capitan Limestone consisting of a massive/unbedded reef
limestone (Capitan-Massive Member) and a basinward dipping, bedded biodetrital member (Capitan Breccia
Member) is well exposed in a narrow belt in the southern Guadalupe Mountains and continues in the subsurface
around the northern margin of the Delaware Basin (Harris and Groover 1989).
The formation of the Massive Member of the Capitan Formation has been interpreted in various ways, includ-
ing a barrier reef, a deep-water skeletal wackestone mound and a cement boundstone mound. Principal constitu-
ents of the reef framework were framebuilding heterotroph organisms (sponges,
Tubiphytes
, bryozoans), en-
crusting organisms (
Archaeolithoporella
and microbialite), infilling internal sediment and an amazingly abun-
dant mass of synsedimentary marine cements (-> 1; Fig. 16.12B). The most important constructional element of
the reefs is a micro-framework (e.g.
Tubiphytes, Archaeolithoporella
, sponges), a consortium of low-growing
reefbuilders (coralline sponges, Fig. 16.12A) and marine-phreatic cement (Weidlich and Fagerstrom 1999).
Open surface communities were probably less common than cryptic communities (Wood 1996). The reef was
constructed in part by microbially bound sediments enclosing growth cavities formed by fenestellid bryozoan
fronds and laminar sponges and strengthened by extensive synsedimentary carbonate cements and biogenic
encrustations. Postmortem biogenic encrustations and extensive early cementation are regarded as major con-
trols on the growth and preservation of the reef structure. Some of the sponges were 'upside down' hanging in
pendant fashion within crypts. Most reef growth is believed to have taken place not in shallow but in deeper
water below the wave base (Saller et al. 1999).
1
'Cementstone'.
Marine cement is volumetrically the most significant component through much of the Massive-Member
of the Capitan Formation. The rock consists predominantly of submarine, originally aragonitic radial-fibrous cement (C)
formed synchronously with biogenic crusts (
Archaeolithoporella
, arrows) growing on sponges (SP) as well as on ce-
ments. Note the botryoids composed of cement fans. Botryoids may comprise more than 90% of reef rock samples. The
cementation took place at or near the shelf margin and has greatly reduced primary porosity. Late Permian (Upper Capitan
Limestone, Capitan-Massive Member): Mouth of Walnut Canyon.
2
Carbonate cement and encrusters
. Synsedimentary intergrowth of
Tubiphytes
Maslov (see Sect. 10.2.6.2) and originally
aragonitic botryoidal cement within a growth cavity. Reef biota exhibit distinct internal zonation patterns, reflecting
changing environmental conditions passing up the slope and onto the shelf. Same locality as -> 1.
3
Cement.
Botryoidal carbonate cement forms a great part of the reef rock. The thin section shows primary fibers of calcite
paramorphic after marine aragonite. Note the plane straight sides of the crystals (see Fig. 16.12B). On the basis of analo-
gies with Holocene reef cements, this calcite cement is interpreted as the diagenetic equivalent of a submarine, aragonite
precursor (see Sect. 7.1.5.2). Massive marine cementation is largely confined to the reef facies and the immediate near-
back-reef grainstones. Same locality as -> 1.
4
Backreef facies.
Recrystallized green algal packstone. Most grains are dasyclads (
Mizzia
; arrow points to a longitudinal
section of
Connexia
, C, an alga known up to now only from the western Tethyan Permian) and peloids, associated with a
few gastropods (bottom left).
Mizzia
grainstones, packstones and wackestones occur in a backreef position (outer shelf
lagoon) immediately shelfward of the Capitan reef. The stands of the dasyclads probably formed in hypersaline lagoons.
Mizzia
limestones represent a worldwide Middle and Late Permian shelf facies. Standard Microfacies SMF 18-D
ASY
. Late
Permian (Tansill Formation, coeval with the Uppermost Capitan Formation): Guadalupe Mountains, Texas, U.S.A.
