Interregional Unconformity on the Top Quartzite Unit of the Lalun Formation (Lower and Middle Cambrian Boundary): Evidance Indicating that the Top Quartzite is not the Base Quartzite of the Mila Formation

The Lower Cambrian Lalun Formation includes the lower Sandstone, middle Shale and upper Top Quartzite units. The contact of the Shale unit with that of the Sandstone unit is unconformable grading upward to Top Quartzite unit. Two bioherm horizons are present near the base of the shale unit. The Shale unit represents deposition in an estuary environment. The Top Quartzite of the Lalun Formation records deposition in a shoreface depositional setting. Facies and depositional environment of the Hawke Bay Quartzite of the Appalachian Mountains and the Zabriskie Quartzite of California are similar to the Top Quartzite of the Lalun Formation. The upper contact of the Hawke Bay and Zabriskie Quartzites is a regional unconformity signifying the Lower and Middle Cambrian boundary. These deposits, as well as, the Top Quartzite are correlated to similar deposits in Turkey, Oman, China, southwest Europe and south of Australia. They are unconformably overlain by Middle Cambrian rocks, too. Therefore, the unconformity of the top of the Top Quartzite unit is interregional, coinciding with the Lower and Middle Cambrian boundary.Two biostromal and biohermal thrombolite horizons present near the base of Member 1 (2 meters above the boundary) indicating the onset of Middle Cambrian transgression. This event resulted in the formation of the transgressive Mila succession (Members 1-3). Therefore, the Top Quartzite of the Lalun Formation does not belong to the Mila succession and cannot be considered as its Base Quartzite unit, as some geologists have previously suggested.The contact between the Shale and Top Quartzite units is gradational and the two units are bounded by two erosional unconformities. Therefore, the Top Quartzite cannot be considered as a separate lithostratigraphic unit.