Loyalhanna Formation
Under the guidance of Dr. Carol de Wet
Department of Geosciences, Franklin and Marshall College, Lancaster PA
Under the guidance of Dr. Carol de Wet
Department of Geosciences, Franklin and Marshall College, Lancaster PA
The Loyalhanna Formation is a Mississippian sandy limestone that has high economic value in many industrial uses. This study focused on the lithology and diagenetic features of the Loyalhanna Formation using petrographic analyses and some whole rock geochemistry analyses, such as LOI, XRD and XRF in order to understand the depositional environment and diagenetic history of the Loyalhanna Formation underlying the Wilhelm Property in Somerset County, Pennsylvania. It is found that the Loyalhanna Formation is produced by nearshore mixing of locally produced marine carbonate grains, aeolian sand and dolomite grains, and fluvial quartz sand delivered from eastern prograding delta facies and newly uplifted western Pocono Formation. The depositional environment of the Loyalhanna Formation is a high energy subtidal marine embayment. The source of dolomite grains is hypothesized to be the erosional upper limy units in the Rockwell Formation that is below the Pocono Formation. The remaining lower unit of the Rockwell Formation in the northwestern Pennsylvania is now named as the Shenango Formation and it has a large upper unconformity. The Loyalhanna Formation has two distinct facies, calcareous facies and shale facies. The calcareous facies was deposited in the embayment and the shale facies was deposited on the lee side of the sand dunes as a result of accumulation of suspended clay deposits in the ocean. The diagenetic history of the Loyalhanna Formation is complex. It starts with syndepositional non-ferran calcareous cements in the pore spaces to lithify the aeolian and fluvial sand and dolomite grains. With the fluctuation of sea level, fibrous meniscus intermediate ferroan calcite crystallized around the large fluvial sand bars in the vadose zone. Due to extensive burial and compaction, some secondary calcite formed and crystallized around the sand grains. During the late stages of burial, groundwater intrusion dissolved the original non-ferroan calcite and reprecipitated as pale blue ferroan calcite. There are still many questions about the history of the Loyalhanna Formation, so further research should be examined.