PETROLOGY AND THE DIAGENETIC HISTORY OF THE CAMBRIAN DUNHAM DOLOMITE, NW VERMONT

This research proposes the study of petrology and diagenetic history of a particular Cambrian Dunham Dolomite sample from northwestern Vermont. The investigated sample consists of black, fine-grained, thinly laminated, sucrosic calcite being intruded by white, coarse-crystalline dolomite. During the original deposition of carbonates, feldspars were simultaneously precipitated, and then the entire mass was subjected to burial diagenesis.

Three thin-sections were made for analysis with the resources at the Indiana University Rock Lab. The petrographic characteristics in the thin-sections indicate that “saddle” dolomites make up the coarse-crystalline portion of the Dunham Dolomite. The coarse-crystalline dolomite crystals are curved or bent and physically forced the fine-grained, sucrosic calcite apart during precipitation. Optical analysis also indicates the existence of feldspar crystals. Many of these feldspars are replaced by carbonates. Some of those feldspars have twinning, while others are in the form of rhombohedra. The rhombic shape indicates that the feldspars are potassium-rich (adularia). Study by electron microscopy is currently being done to confirm that the rhombic feldspars are indeed adularia.

Analysis using X-ray diffraction (Bruker AXS D8 Diffractometer) verified that the fine-grained, sucrosic mass was calcite (~90%) instead of dolomite. The X-ray diffraction pattern shows minor amounts of quartz, micas, feldspars, and trace amounts of smectite. Further research and the use of acid to get rid of the carbonates will result in a higher concentration of feldspars. Analyzing this acid-treated sample with the Diffractometer will give a better result of what type(s) of feldspar are present.

The hand sample exhibits multiple micro fractures and micro faults resulting from the physical displacement of the sucrosic calcite by the coarse-crystalline dolomite. These micro fractures and micro faults fit perfectly back together giving another indicator as to the history of the sample. Many stylolites can be seen in the hand sample due to pressures exerted upon the sample. When viewed under the microscope these stylolites are “fuzzy” indicating that they had formed before diagenesis.