Page 285 - Carbonate Facies in Geologic History

272 Reef Trends and Basin Deposits in Late Jurassic Facies

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Fig. IX-6. Sponge mounds in Kimmeridgian beds of Schwabia from near Miinsingen. From
Gwinner (1962). Mound is micritic with scattered lithistid sponges. Bedded limestones onlap
flank. The height of the mound at end of growth was at least 50 m

Kimmeridgian; it now extends under the Molasse plains to the south. The indi-
vidual mounds show some intergrading and interbedding of massive and nor-
mally bedded limestone and marls. In places, however, they accumulated as very
large masses of rapidly accreted and uncompacted carbonate. In these mounds a
sea floor relief of as much as 100 m may be demonstrated by observing strati-
graphic onlap against the sides of the buildups. The slopes of individual mounds
average 10-15 degrees but may be much steeper (Fig. IX-6).
The constituent sponges are flat or cup-shaped siliceous forms now largely
replaced by crystalline calcite; they occur in a micrite matrix. The total mega-
fauna, considered in variety of species, consists of 8-10% sponges, 10% bryo-
zoans, 20% brachiopods, 35% serpulids and other encrusting forms, and 13%
anmonites, various bivalves, belemnites, etc. Volumetrically, the bulk is sponges
with abundant encrusting algae and tubular foraminifera, the invertebrate shelly
fauna consisting of small forms inhabiting the underside of sponges. Abundant
calcareous crusts coating the sponges are presumed to be symbioses of alga-
nubicularid foraminifera; they preferentially grew upward (like stalagmites) to
form knobs and pellets (Hiller, 1964). Additionally, algal? stromatolites in the
higher mounds encrust in place of the alga-tubular foraminifera association.
These sponge-algal mounds are interpreted as developing in quiet and some-
what deeper water than one normally associates with carbonate buildups for the
following reasons:
1. Relief of as much as 100 m can be stratigraphically demonstrated by inter-
reef fill-in against sides of mounds; depths of interreef areas of at least this much
must have existed at the end of mound accumulation and possibly at its begin-
mng.
2. Equivalent off-mound beds contain even alterations of marls and lime-
stones and a glauconite bed that can be widely traced across the Schwabian Alb-
essentially indicating sedimentation below wave base.
3. Micritic matrix of mounds indicates quiet-water deposition.
4. Siliceous sponges commonly grow at considerable depths.
5. There is little or no evidence of extensive boring organisms.
6. There is no talus or widespread bioclastic debris surrounding the mounds.
7. The tops of these mounds were colonized by algae but not by corals. They
were, therefore, in the photic zone but below wa ve base. As much as 50 m of water
may have existed above the mounds.