Sandy Substrate
171
holes into calcareous rock can make up some 30 % of the sediment in certain lagoons
of the Pacific coast.
In shallow rocky areas, organisms commonly are removed periodically by wave
action, and their remains collect in depressions or at the foot of the rocks. The
reef-talus deposits are of this origin. They are coarsely layered thick sequences of
sediments surrounding the reef structure, and are familiar from ancient exposed reefs
on land (Permian EI Capitan Reef, west Texas; Triassic Dolomites in the Alps;
Jurassic MaIm reefs in middle Europe; Cretaceous Albian reefs in southern Arizona).
The coarse-grained material of the reef talus is highly porous and permeable, and can
serve as reservoir rock for petroleum - as in the oil fields of the Middle East.
We shall return to reef formations in Chapter 7 when discussing the significance
of marine deposits as climate indicators.
6.4 Sandy Substrate
Rocky ground quite commonly has a rich, "flashy" assemblage of diverse organisms,
from tropical reefs to Antarctic sponge forests (Fig. 6.11). Substrate of pebbles and
boulders is much like rocky bottom provided there is no movement. If waves move
the rocks, most sessile benthos cannot survive.
The benthic communities on sandy sea floor are much less flashy. Commonly one
sees rather little - the organisms are mostly hidden within the sand. The reason why
sessile epifauna is largely absent is the instability of the substrate. Where it is stable,
sea grass can take hold and further stabilize the ground. Epibenthic diatoms, foraminifera, and bryozoans grow on such grass.
Vagile benthos, with or without burrows to return to, can be quite abundant on the
sandy bottom. Crabs and snails are common sights. In the intertidal range there is a
large variety of nonmarine invaders during low tide, mainly birds. These hunt for the
hidden infauna. Conversely, during high tide the invasion is from the sea: sting-rays
and other fishes digging up worms and mollusks.
The presence of so many predators, and the shifting of the sand which can suddenly expose the infauna, require the ability to burrow very rapidly. Crabs demonstrate this adaptation very obviously, but also many clams can burrow quickly - as
clam diggers well know. Burrowing clams have a strong long foot which they extend
into the sand below, then inflate by water pressure to anchor it, and pull the rest of the
body down through muscle contraction. A smooth outer shell, usually quite sturdy,
characterizes such clams. The study of burrowing mechanisms and the resulting
tracks in the sediment is a research field by itself (Sect. 6.6).
Burrowing clams are suspension feeders, with an inhaling and an exhaling siphon
(Fig. 6.12). After death, the shells of such bivalves - hydraulically quite different
from the surrounding sand - are sorted out and concentrated into layers of coquina
(Fig. 6.13). Such coquina deposits are widespread in the geologic record and can be
used as marker beds locally.
171
holes into calcareous rock can make up some 30 % of the sediment in certain lagoons
of the Pacific coast.
In shallow rocky areas, organisms commonly are removed periodically by wave
action, and their remains collect in depressions or at the foot of the rocks. The
reef-talus deposits are of this origin. They are coarsely layered thick sequences of
sediments surrounding the reef structure, and are familiar from ancient exposed reefs
on land (Permian EI Capitan Reef, west Texas; Triassic Dolomites in the Alps;
Jurassic MaIm reefs in middle Europe; Cretaceous Albian reefs in southern Arizona).
The coarse-grained material of the reef talus is highly porous and permeable, and can
serve as reservoir rock for petroleum - as in the oil fields of the Middle East.
We shall return to reef formations in Chapter 7 when discussing the significance
of marine deposits as climate indicators.
6.4 Sandy Substrate
Rocky ground quite commonly has a rich, "flashy" assemblage of diverse organisms,
from tropical reefs to Antarctic sponge forests (Fig. 6.11). Substrate of pebbles and
boulders is much like rocky bottom provided there is no movement. If waves move
the rocks, most sessile benthos cannot survive.
The benthic communities on sandy sea floor are much less flashy. Commonly one
sees rather little - the organisms are mostly hidden within the sand. The reason why
sessile epifauna is largely absent is the instability of the substrate. Where it is stable,
sea grass can take hold and further stabilize the ground. Epibenthic diatoms, foraminifera, and bryozoans grow on such grass.
Vagile benthos, with or without burrows to return to, can be quite abundant on the
sandy bottom. Crabs and snails are common sights. In the intertidal range there is a
large variety of nonmarine invaders during low tide, mainly birds. These hunt for the
hidden infauna. Conversely, during high tide the invasion is from the sea: sting-rays
and other fishes digging up worms and mollusks.
The presence of so many predators, and the shifting of the sand which can suddenly expose the infauna, require the ability to burrow very rapidly. Crabs demonstrate this adaptation very obviously, but also many clams can burrow quickly - as
clam diggers well know. Burrowing clams have a strong long foot which they extend
into the sand below, then inflate by water pressure to anchor it, and pull the rest of the
body down through muscle contraction. A smooth outer shell, usually quite sturdy,
characterizes such clams. The study of burrowing mechanisms and the resulting
tracks in the sediment is a research field by itself (Sect. 6.6).
Burrowing clams are suspension feeders, with an inhaling and an exhaling siphon
(Fig. 6.12). After death, the shells of such bivalves - hydraulically quite different
from the surrounding sand - are sorted out and concentrated into layers of coquina
(Fig. 6.13). Such coquina deposits are widespread in the geologic record and can be
used as marker beds locally.