Ninth international conference on - Marum

92
Abstracts of posters
5 survey lines (Fig. 1) have been repeated at a total of 10 times at various times of year, at high and low tide;
water depth, temperature and salinity, and atmospheric pressure variations have been recorded in each case.
Although sub-seabed conditions are generally characterised by muddy sediments, the survey line closest to the
Rande Strait is crossed by two channels in which there are coarser sediments (muddy silts). Salinity is affected
by the introduction of fresh water from the Redondela River. Atmospheric pressure, water depth and sub-seabed
depth have been used to calculate the hydrostatic pressure at the gas front.
The data presented in Figure 2 represent extreme cases in which only one of the main parameters varies:
1) salinity variations have little or no effect; a change from 18.95 to 32.89‰ was accompanied by 4 cm
(average) change in the depth of the gas front.
2) a change in hydrostatic pressure from 109 to 135 kPa (mainly caused by a 2.7 m change in water depth)
was reflected by a gas front movement of 10 cm (average).
3) the greatest change (13 cm average) was associated with a rise 4.2º in seawater temperature.
Fig. 2: Three pairs of survey lines, each showing a significant variation in 1 of the 3 parameters: temperature
(∆T), hydrostatic pressure (∆P) and salinity (∆S). Corresponding changes in gas front depths can be seen in the
profiles and the average change in gas front depth (∆Dg).
Although the water temperature had the greatest influence on the depth of the gas front, it is interesting to note
that the gas front became closer to the seabed when the temperature was higher in the muddy sediments, but
actually fell at those locations where the seabed sediment contains a greater proportion of gravel-sized particles
(mainly shell material) suggesting that the sediment texture also has some influence.
Miocene seep-carbonates as indicators of style and intensity of fluid migration
S. Mecozzi 1 , S. Conti 1 & D. Fontana 1
1 Department of Earth Sciences of the University of Modena and Reggio Emilia
Miocene seep-carbonates have been reported from marine sedimentary successions of the northern Apennines.
They are recognized by their peculiar palaeoecological, sedimentological, compositional and isotopic features as
products of the microbial oxidation of methane-rich fluids and represent an excellent example of carbonate
bodies interpreted as the remains of ancient cold seeps.
These seep-carbonates occur from internal tectonic zones (Piedmont Terziary basins, epi-Ligurian and minor
basins) to external zones of the foredeep. In the Miocene foredeep, they crop out in large turbiditic bodies (Mt.
Cervarola and Marnoso-arenacea Formations) and in slope hemipelagites (Vicchio and Verghereto Marls, and
Ghioli di letto mudstones). Dominant rock types are calcilutitic/marly limestones, calcareous marls and
calcarenites. Enclosing sediments are hemipelagic/turbiditic mudstones, muddy sandstones and marlstones.
In the Apenninic chain, the abundance and the extent of the outcrops provide a rare opportunity to study the
geometry, facies distribution and internal structures of fossil methane-derived carbonates.
On the basis of morphological and stratigraphic features two main types of seep-carbonates were distinguished in
the field (type 1 and 2).
The type 1 is composed of a horizontal repetition of decametric to heptometric carbonate bodies, lenses and
pinnacles. They have a thickness of 5 - 30 m and an extension that ranges from 10 m to 100 m. The basal
portions of these huge bodies are strongly brecciated, made up of intraformational polygenic breccias and rarely
extraformational. The Sasso Streghe (Figure 1, Modena Apennines) and Monte Petra (Romagna Apennines)