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Palynofacies of the epicontinental Lower Jurassic deposits of Poland: their distribution in different sedimentary environments

Pieńkowski G., Waksmundzka M. M.

Volumina Jurassica

2006

Vol. 4, no. 1

102-103

Diversified continental, marginal-marine and marine epicontinental deposits in Poland of Early Jurassic age yielded unusually rich palynofacies. 210 samples from 6 selected boreholes have been studied in order to determine distribution of palynomorphs and other kerogen particles in various depositional systems, prior determined by detailed sedimentological studies. Several palynofacies parameters, like content of terrestrial phytoclasts, their type and degree of oxygenation, spore/bisaccate pollen ratio, presence of tetrads and sporangia, presence of amorphous organic matter - AOM (both of terrestrial - AOMT and aquatic - AOMA origin), presence of acritarchs and dinoflagellate cysts were collectively found as indicative of certain palynofacies. Generally, six main palynofacies linked to certain depositional systems have been distinguished: 1. alluvial plain - very abundant palynological matter, varied frequency of sporomorphs, varied bisaccate pollens/spores ratio (depending on a local vegetation), very abundant dark-brown to opaque phytoclasts or AOMT (amorphous organic matter of terrestrial derivation); 2. delta plain - abundant dark to opaque phytoclasts (or AOMT) and cuticle, abundant sporomorphs (spores dominate over bisaccate pollens), sporadic presence of dinoflagellate cysts and acritarchs; 3. delta front-lagoon – abundant sporomorphs, varied bisaccate pollens/spores ratio (usually spores and pollens are in approximate ballance, but sometimes huge number of spores accompanied by tetrads and sporangia occurs in “hydrodynamic traps”), moderate amount of phytoclasts - mostly cuticle, relatively rare dark phytoclasts, rare dinoflagellate cysts and acritarchs;4. shoreface-foreshore - dispersed and rounded fragments of black, opaque phytoclasts or AOMT, rare to moderate sporomorphs (bisaccate pollens and spores in ballance or slight dominance of spores), sporomorphs often mechanically destroyed, rare dinoflagellate cysts and acritarchs; 5. offshore brackish marine - rare translucent AOMA (amorphous organic matter of aquatic derivation), very rare small and rounded fragments of opaque phytoclasts or AOMT, rare sporomorphs (bisaccate pollens dominate over spores), occurrence of dinoflagellate cysts and acritarchs; 6. offshore marine - translucent AOMA (amorphous organic matter of aquatic derivation), rare bisaccate pollens, spores absent or very rare, occurrence of dinoflagellate cysts and acritarchs. Transitional palynofacies between these main types can be also distinguished, but it must be coupled with sedimentological data. Some abnormal composition of palynofacies have been observed and they are defined as palynofacies inversions. The first type (type I) of inversion represents coexistence in one sample of palynomorphs of the same geological age but of drastically different colours, which is explained by darkening of some palynomorphs by intraformational oxidation processes. The second type (II) of inversion occurs when characteristic alluvial/deltaic palynofacies elements (like sporangia, tetrads, large phytoclasts) are found together with obviously offshore elements (light AOMA, dinocysts, accompanied by ammonites and open shelf organisms). This inversion is explain by a storm resuspension processes. Abnormally high number of spores observed in few samples of an interdistributary lagoon origin is explained by hydrodynamic entrapment of spores. Some regular fluctuations of palynofacies characteristic within lagoonal/lacustrine laminites are described as a result of seasonal changes. Collectively, palynofacies can be regarded as useful tool for recognition of palaeoenvironments. However, palynofacies studies should be coupled with sedimentological investigations.