Features and origins of massive dolomite of Lower Ordovician Penglaiba Formation in the northwest Tarim Basin: Evidence from petrography and geochemistry

An integrated petrographical and geochemical study of the massive dolomite of the lower Ordovician Penglaiba Formation of the Tarim Basin, outcropping at Yonganba recognized three dolomite types: very finely to finely crystalline nonplanar-a to planar-s dolomite (D1); medium crystalline planar-s to planar-e dolomite (D2); and coarse crystalline nonplanar-a dolomite (D3). All have been affected by burial. D1 and D2 dolomites developed initially before or during shallow burial and later recrystallized, whereas D3 dolomite replaced the initial limestone entirely during burial. All three dolomites have similar geochemical features. The D2 dolomite tends to have more inter-crystalline pores (inherited from primary pores) and higher porosity due to its outstanding compaction resistance during shallow burial; whereas D3 dolomite does not retain appreciable primary pores due to strong cementation and pressure dissolution before dolomitization. This study provides a useful model for understanding the origin and porosity development of burial dolomite, in particular Paleozoic dolomite.