EVALUATION OF TERRESTRIAL AND OFFSHORE HEAVY MINERAL ASSEMBLAGES IN DELAWARE FORMATIONS

Heavy mineral samples from Pliocene and Pleistocene offshore deposits, onshore outcrops, and modern beaches were analyzed to determine: (1) the extent to which on/offshore heavy mineral suites from the same formations vary and (2) the extent to which heavy minerals can be used to differentiate among formations. Eight 15-20 ft vibracores collected from offshore Delaware were divided into 5 ft sections for analysis and seven onshore Pleistocene outcrops and modern beaches were sampled. Samples were wet sieved to isolate the sand fraction (grains < 2.00mm and > 63 μm) and spiraled using a 3-turn Humphrey Spiral to produce a heavy mineral sand concentrate. Actlabs determined the weight percent of each mineral by point counting a portion of each concentrate.

Data collected by Jordan (1964) from terrestrial exposures were used in the analysis; his formation assignments were updated in summer 2018. Jordan (1964) collected and sieved 75 samples to obtain grains in the 500 to 62 μm range. The heavy minerals were separated with tetrabromoethane and mounted to identify ≥ 100 non-opaque grains. Those results were converted to Actlab compositions to compare the results to the data collected in 2017. The heavy mineral suite in Jordan’s (1964) samples was dominated by opaques (ilmenite, magnetite, and leucoxene; average: 77.6%), zircon (average: 6.8%), pyribole (amphibole and pyroxene; average: 4.0%), and epidote (average: 3.9%).

Preliminary results indicate that the on/offshore mineral assemblages of the Beaverdam Formation are highly variable. The offshore samples have greater epidote (8.3% vs. 4.0%) and less zircon (3.7% vs. 8.1%). A one-way ANOVA test found the abundances of several minerals including zircon, garnet, and staurolite differed significantly among the Columbia, Lynch Heights, Omar, and Beaverdam Formations. An independent t-test comparing Jordan’s (1964) 21 Columbia Formation samples, 33 Beaverdam Formation samples, and data collected in summer 2017 shows a significant difference only among opaques, monazite, and zircon. The abundance of each mineral is variable within each of these two formations, indicating that heavy minerals cannot be used to discriminate between these formations.