a conglomerte bed as a possible lower boundary - Professor Kamal ...

CHAPTER TWO
STRATIGRAPHY AND SEDIMENTOLOGY
hemisphere of the earth and consequently sea level rise. The deposition of limestone
started in the opposite situation to that of chert. The marl or shale is deposited at the
intermediated distance. Hori et al., (1993) ascribed the occurrence of chert/shale
pairs (couplet) in Franciscan Formation to Milankovitch climatic cycle (~100,000
years cycles of warm and cold periods). During the warm periods, the radiolarians
bloomed and their skeletons rained into the ocean floor. In colder periods, the
radiolarians were scarces and clays transported by ocean currents or winds were
deposited.
2-6-Origin of the Chert in the Qulqula Formation
2-6-1-Origin of Chert Nodules
Qulqula Formation is characterized by its bedded chert content and even the
limestone beds contain different types of chert nodules and strings (Fig.2.20). These
nodules are random and have not any observable cyclicity. The chert nodules show
displasive habit and diagenetic origins, which manifested by four points:
The first one is that some nodules are found in cross laminated sandy limestone
(lithoclastic or peloidal grainstone). The nodules cut the laminae of limestone bed
(Fig.2.21). Therefore, when the principle of “cutter is younger than the cut” is used
the nodules become younger than limestone. In the other samples, strings
(discontinuous narrow belt) of the black chert are found in the sandy limestone of the
limestone succession of the lower part of the formation.
The second point is that the nodules are found also in limestone with trace fossils
(Fig.2.22).
The third point is that under stereoscope microscopes, the nodules show
displasive habits, which pull apart the limestone material away from the boundary of
the nodules.
The fourth point is that under stereoscope microscopes the transitional stage of
silicification (partial silicification) of peloidal and bioclastic limestone can be seen. In
these partially silicified rocks, the original constituent can be found such as crinoidal
fragment, which has net texture of the crinoids, but it loss its famous syntax
extinction. These features of relations between chert nodules and host limestone
prove that they formed after deposition of the limestone by diagenesis. Most possibly
they formed after or during burial of the sediments.
Replacement and displaced growth of chert nodules in Qulqula Formation require
concentration of silica ions in certain locus within the limestone. The diffusion and
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