SEDIMENTOLOGY AND PALEOENVIRONMENTAL RECONSTRUCTION OF THE LOWER PORTLAND FORMATION (HARTFORD BASIN, NEWARK SUPERGROUP)

The Hartford Basin of Connecticut is a Mesozoic rift basin formed during the opening of the Atlantic Ocean, containing sedimentary and igneous fill of the Newark Supergroup. The Portland Formation is the youngest formation (Sinemurian-Toarcian) within the basin, 25 wireline rock cores penetrating the lowermost Portland Formation were logged and sampled to better understand facies architecture and paleoenvironments within the poorly exposed center of the basin. These purely sedimentological interpretations are supplemented by both biomarker analysis and powdered XRD diffraction. Volumetrically dominant facies include: laminated black mudrock, ripple cross-laminated gray-black mudrock, disrupted black-gray mudrock, laminated red-gray mudrock, ripple cross-laminated red mudrock, disrupted to massive red-gray mudrock, ripple cross-laminated to trough cross-bedded sandstone, planar bedded sandstone, and laminated muddy sandstone. Interpreted to represent sedimentation in three general paleoevironments: sandflat, playa to playa-lake, and perennial saline lakes, thin but correlative breccia, limestone, and ash beds occur rarely.

Lacustrine mudrocks are the most laterally continuous facies encountered in the lower Portland Formation and have the largest impact on basin history reconstruction. Ten continuous black mudrock units were correlated across the cored interval based on general stratigraphy, and the continuity of tufa-like limestone units. These mudrocks contain up to 30% carbonate in the <4 micron fraction. Carbonate clasts or cement within mudrocks transitions from ankerite to dolomite/magnesite-dominated up section. Reconciling patterns in the clay mineralogy of mudrocks as allogenic or autogenic requires interpretation. However, large increases in the percentage of macroscopic evaporite textures in playa facies and abundant analcime in lacustrine mudrocks suggests that balanced-to-closed lake drainage conditions persisted well into Portland time. Lateral heterogeneities in the textures and clay mineralogy of these mudrocks among cores, imply that lake-margin and perennial lake environments are diverse subenvironments with spatial and temporal variability as well as incongruous lateral continuity, further obscured by possibly distinct diagenetic provinces.