hypotheses for bedded cherts

Seven hypotheses proposed for the origin of bedded radiolarian cherts:
(after Hori, R.S; Cho, C; and Umeda, H., 1993, Origin of cyclicity in Triassic-Jurassic radiolarian bedded cherts of the Mino accretionary complex from Japan, The Island Arc, 3, 170-180)

Diagenetic Process	Deposition of clay-Radiolarian mixture with	1. Post depositional separation of clay and silica of Radiolarians.
Depositional Processes	Deposition of clay-Radiolarian mixture with	2. Silica compensation depth (SCD) perturbation removes Radiolarians through dissolving.
	Deposition of clay-Radiolarian mixture with	3. Turbidites of clay-Radiolaria mixture and consequent sorting.
	Constant deposition of Clay with	4. Episodic and rapid deposition of Radiolaria.
	Constant deposition of Clay with	5. Turbidites of Radiolaria.
	Constant deposition of Radiolaria with	6. Episodic and rapid deposition of clay.
	Constant deposition of Radiolaria with	7. Turbidites of clay

Notes:

The absence of CaCO₃ in the Franciscan cherts suggests that they were formed below the carbonate compensation depth (CCD).

The fourth hypothesis may be supported by sediment accumulation rates for the Franciscan chert/shale pairs suggesting that the Milankovitch climatic cycle (~100,000 years cycles of warm and cold periods) could explain the pairings. In this hypothesis, during the warm periods, the radiolarians thrived and their skeletons rained onto the ocean floor. In colder periods, the radiolarians were scarce and clays transported by ocean currents or winds were deposited.