Page 292 - Carbonate Facies in Geologic History

Interpretations 279

Interpretations

Explanations of the Solnhofen environment have varied. Prior theories include
derivation of the mud from the northern coastal regions as windblown dust or
from restricted marine origin in backreef shallow platforms and lagoons. Most
evidence indicates a southern source and an origin as fine detritus from the reef
barrier mixed with pelagic coccolith ooze. Buisonje (1972) proposed that sudden
death of coccoliths due to "red tides" caused great floods of fine carbonate mud.
Other theories have postulated deposition on tidal flats with mud made sticky by
periodic drying under subaerial exposure. Lack of the characteristic and ubiqui-
tous sedimentary structures of the tidal flat preclude this interpretation. The
following theory of origin is derived from Barthel's (1970) discussion.
1. Deposition occurred in small partly isolated basins which originated by
infill of a sea floor relief of 30-60 m between sponge-algal reefs; the top of these
bioherms may well have been below wave base and thermocline, an estimated
minimum of 20 m water depth.
2. The land mass to the north uplifted, and as water shallow ed, rapid growth
of coral reefs occurred, colonizing some of the sponge-algal mounds, accentuating
the basin-swell topography and most importantly forming a southern barrier
cutting off the basins.
3. Carbonate sediment was derived principally from plankton in the open sea
to the south and as fine detritus from the reef barrier.
4. Accumulation took place in a hot, seasonally evaporative climate as indi-
cated by contemporaneous anhydrite and salt in northern Germany, Paris, and
Moroccan coastal basins. Flora and fauna also indicate a warm, subtropical
climate; there is an abundance of cycads, reptiles, insects.
5. The basins became stagnant with hypersaline brines, derived from evapora-
tion of inflowing sea water mixed to some extent with fresh water from land to the
north. The brine flowed out continuously through the reef barrier and never
reached the necessary salinity to precipitate sulfates (100-150 ppm). It was, how-
ever, sufficiently saline to kill and preserve (pickle) organisms in a largely anaero-
bic environment.
Several interesting questions appear unanswered in regard to the Solnhofen
facies. Why was there insufficient organic matter to color the sediment black,
particularly in view of the known subtropical climate on the land to the north?
How did the bottom mud become cohesive enough to retain prints?
Modern lagoons with photic zone bottoms contain a soupy layer of blue-green
filaments of organic slime and aragonite-sediment insufficiently coherent to be
grooved. The Solnhofen lagoons indicate very early consolidation of bottom mud,
a process which has not taken place in modern shallow water muds since their
formation during the several thousand years since post-glacial sea-level rise.
Compaction of the Solnhofen lime mudstone is greater than one would expect by
comparison with modern shallow-water lime muds. The estimated 4 to 6 times
volume compaction of laminae away from early ca1citized fossils and the flatten-
ing of many organisms seems too high for pure ca1cilutites.
This Chapter has concentrated on European shelf marginal carbonate build-
ups and associated lagoonal facies. In addition, Jurassic shelf strata, the world