Naslov
The repetition of the cycles - another autocyclic explanation

Autori
Bucković, Damir

Vrsta, podvrsta i kategorija rada
Sažeci sa skupova, sažetak, znanstveni

Izvornik
3. Hrvatski geološki kongres : Knjiga sažetaka : Abstracts Book / Velić, Ivo ; Vlahović, Igor ; Biondić, Ranko - Zagreb : Hrvatski geološki institut, 2005, 19-20

ISBN
953-6907-07-0

Skup
3. Hrvatski geološki kongres

Mjesto i datum
Opatija, Hrvatska, 29.09.2005. - 01.10.2005

Vrsta sudjelovanja
Poster

Vrsta recenzije
Domaća recenzija

Ključne riječi
Middle Lower Jurassic; autocyclicity; Adriatic-Dinaric carbonate platform; Mt Velika Kapela; Croatia

Sažetak
Middle Lower Jurassic carbonate succession of Mt Velika Kapela is composed of coarsening-upward cycles. Analysis of facies alternation and stacking pattern reveals a sporadic appearance of oolitic facies indicating oolitic shoal progradation from neighboring topographic highs. These progradations interrupted the constant "in situ" carbonate sediment aggradation, which produced typical coarsening-upward architecture. Two types of coarsening-upward cycles can be distinguished: (1) cycles with peloidal-bioclastic upper cycle members, and (2) cycles with oolitic upper cycle members. It can be presumed that these facies differences, i.e. cycle architecture, are mainly the result of autocyclic processes operating within the Adriatic-Dinaric carbonate platform (ADCP). Among those processes, aggradation of subtidal carbonate deposits and progradation of small ooid shoals appear to be most important. During periods when the water depth (i.e., accommodation space) on many ADCP parts was greater than one-half of the wavelength, there was no movement of sediment particles at the sea bottom. Therefore, in quiet subtidal environments below the fair-weather wave-base, peloids and calcareous mud, as well as benthic forams occasionally associated with various bioclasts, were predominantly accumulated, producing muddy carbonate deposits ("catch up" phase sensu KENDALL & SCHLAGER, 1981). It is presumed that the carbonate production rate exceeded both the rate of subsidence and the rate of eustatic sea-level rise (i.e. relative sea-level rise). Such high carbonate accumulation rate caused gradual reducing of accommodation space, i.e., decrease of water depth. When the water depth became less than one-half of the wavelength, there were occasional motions of sediment particles by waves on the sea-bottom. Under such shallower-water and higher energy conditions, various coarser-grained calcareous particles were formed, producing grain supported deposits. With continuing carbonate production, accommodation space further decreased and the sea-bottom continues to aggrade ("keep up" phase sensu KENDALL & SCHLAGER, 1981). Yet, such environment with agitated water, supersaturated with CaCO3, became unfavorable for the majority of carbonate-producing benthic organisms (e.g. foraminifera, mollusks). Also, highly oxygenated pore waters and high water exchange rates enhanced the process of cementation. Under such conditions, the rate of new carbonate sediment production slowed down, i.e. the carbonate accumulation was easily outpaced by continued relative sea-level rise, and sea-bottom was lowered below the fair-weather wave base. After this "lag phase" (READ et al., 1986 ; SCHLAGER, 1992), carbonate-producing organisms (mostly mass faecal pellets producers, e.g. worms, ostracodes, gastropods) colonized the sea-bottom drowned below the fair-weather wave base, and carbonate production was re-established ("start-up" phase sensu KENDALL & SCHLAGER, 1981 ; or "lag phase" sensu SCHLAGER, 1992) ; a new cycle with a muddy lower member started to accumulate ("catch-up" phase sensu KENDALL & SCHLAGER, 1981). Such autocyclic mechanism would explain the origin of the coarsening-upward cycles with peloidal-bioclastic upper cycle members. In contrast to that, environmental conditions within sporadically presented subtidal topographic highs were rather different. In these places, the water depth was smaller and the wavelength was consequently shortened, so the sea-bottom was positioned closer to the fair-weather wave-base. Thus, when the water depth became less than one-half of the wavelength, water temperature and pH as well as the carbonate saturation level and water energy increased, favoring the formation of ooids (e.g., GONZALES, 1996). In such conditions, the ooids are in constant motion allowing them to grow around a nucleus forming an oolitic shoal. With continuing production of ooids the accommodation space decreases. When oolitic shoals aggraded close to the sea level, they begin to prograde laterally under influence of tidal and/or storm currents (when distinct cross- and/or horizontal lamination was formed), covering the adjacent deposits below and/or above the fair-weather wave-base and forming the coarsening-upward cycles with distinct upper oolitic and/or oolitic-bioclastic cycle members. Therefore, the occurrence of cycles with ooids reflects the oolitic shoal progradations. These oolitic shoal progradations were random, episodic processes that depended on physical factors working in the sedimentary environment. Among those, the most important was the formation of subtidal topographic highs on which the water depth was reduced. It is presumed that small-scale synsedimentary uplifts created these topographic highs. These small-scale uplifts were random events, with no predictable periodicity and place of occurring. Thus, in periods when small topographic highs existed, oolitic shoals also existed and prograded over the subtidal below the fair-weather wave-base when one coarsening-upward cycle with ooid grainstone as the upper cycle member was formed. Analogously, in periods when oolitic shoals prograded over the subtidal above the fair-weather wave-base, one coarsening-upward cycle with ooid-bioclastic packstones to grainstones as the upper cycle member was formed. In periods when high energy storm events triggered progradation, i.e. rapid migration of oolites, these currents succeeded to erode subtidal bottom when erosive bases of the oolites were formed. During periods without oolitic shoal progradations, only vertical aggradation of subtidal carbonate deposits occurred, producing successive series of coarsening-upward cycles with peloidal-bioclastic upper cycle members.

Izvorni jezik
Engleski

Znanstvena područja
Geologija

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