Page 27 - Maldonado_Stanley_1976

NUMBER 16 23
CoARSE CALCAREous SAND.-The sediments in- ual sediment types where age is taken into con-
cluded in this group are characterized by a high sideration (Emery, 1952, 1968). In many parts of
percentage of calcareous organic fragments, form- the Mediterranean maerl appears to accumulate at
1
ing generally more than 30% and not less than 15% present between the intracoastal compiex and the
of the samp1es. Grain count values as high as 90% inner-outer shelf transition zone. Relict maerl,
are displayed by some samples. The grain size is originating in outcrops on the middle to outer
variab1e, with a median comprised between 0.5 and shelf, developed during the last glacial and the
2.5 mm. The content of fines (silt and day) also is subsequent rise of sea level (Caulet, 1972a, b).
highly variable and ranges from 0.5 to 30%. The Since this rise in sea level, a fine fraction has been
calcareous components are largely molluscs added, thus modifying the aspect of the originai
(mainly gastropods and pelecypods), coralline al- carbonate sediment (cf. discussion in Swift, 1974;
gae, echinoderms, bryozoans, and forams. Kulm et al., 1975). As an example, Milliman et al.
This sediment type has been observed on various (1972:254) have shown that calcareous sediments
other shelves and banks of the Mediterranean Sea o n banks in the Al boran Se a are relict an d orig-
(Dangeard, 1929; Blanc, 1958, 1972; Cau1et, 1972a, inally formed at depths of 70 to 100 m during
b; Milliman et al., 1972; and others). On these lower stands of early Holocene sea level. Residua!
she1ves two main facies are distinguished on the deposits are well differentiated in these calcareous
basis of composition: (l) shelly and bryozoan de- sediments: they display mixed thanatocoenoses
posits and (2) coralline sands. The first type can from different environments, and the various
be differentiated into severa! subtypes which appear components show different states of preservation.
depth reiated: (a) "muddy shelly sands" tend to be The coarse calcareous sands are present in ali
more important as depth increases (Milliman et shallow platform cores of the Strait, and also are
al., 1972); (b) "very coarse shelly sands" (cf., well represented in Strait Narrows cores. Typical
Posidonia facies as described by Blanc, 1972) also shelly coarse sand, abundant in the shallower plat-
may be represented at shallower depths (about form cores, is also found in cores collected in
40 m); and (c) "clean fine-grained shelly sands" are deeper environments (e.g., core SP 8-7, 350 m).
generally found in the nearshore-inner neritic Moreover, some cores in deep basins (e.g., KS-12)
environment. also display this facies; bere the sands bave been
The second facies, the coralline sands, are domi- transported from shallower environments and de-
nated by particles of calcareous aigae, which con- posited downslope by gravity flow mechanisms.
stitute as much as 70% of the sediment. Three main Coralline sands were not recovered in our cores,
subtypes are distinguished on the basis of the cal- but bave been reported from the Strait (Blanc,
careous alga! type: (a) alga! bali facies ("fonds à 1958); they also appear in some bottom photo-
pralines," cf., Blanc, 1958); (b) encrusting cal- graphs (Figure 21; Akal, 1972).
careous red algae, commonly developed on rocky SAND-SILT SIZE SEDIMENTS.-The sand-silt sedi-
substrates; (c) coralline gravel consisting of the ac- ment type is distinguished on the basis of structure,
cumulations of debris of the red calcareous algae texture and, to a !esser extent, composition. This
Lithotamnium calcareum and Lithotamnium cor- type includes sediments which range in size from
allioides, with a subordinate association of mol- sand to fine silt, and includes a host of intermediate
I uscs, bryozoans, an d foraminifera ( this forms the sizes.
sediment type termed "maeri" by French authors; Sand-silt varieties are encountered in the shal-
cf. Caulet, 1972). Iow piatform environment and in the deep basins,
Ali of the calcareous deposits described above but are uncommon to rare in the neritic-bathyal
also may be classified into modern, relict, and resid- cores. This sediment type is generally structureiess
in the shallow platform environment. It is geneti-
1 In this study we cali "bioclastic" grains those clasts of
cally related to the coarse calcareous sand type,
organic origin which display evidence of transport and
and usually shows a gradational transition with
reworking (rounding and shape or other indirect evidence);
grains of organic origin which do not show clear evidence these coarser deposits both laterally and vertically.
of reworking by transport processes are referred to by the The coarser sediment (median to coarse sand) ob-
more generai term, "biogenic." served in deep basin cores dispiays distinct and