Reconstructing landscapes across the early to late Cretaceous transition – evaluating base level, climate and sequence stratigraphy from Potomac formation sediments in New Jersey and Delaware
Creators: name: Thornburg, Jesse Daniel
Publisher: name: No Publisher Supplied
Year: 2016

description: The mid-Cretaceous (Barremian/Aptian/Albian/Cenomanian stages) marks the transition to global greenhouse climatic conditions.  The mid-Cretaceous Potomac Formation, deposited on the North American coastal plain, offers the potential to study a nonmarine fluvial/deltaic deposit that experienced changes in climate and sea level.  This study involves three coreholes from New Jersey (Fort Mott and Medford) and Delaware (Summit Marina) that were used to evaluate the landscape evolution through this time interval and to develop an enhanced method of correlation between these sites.   Paleosols offer excellent records of terrestrial conditions during their formation.  103 total paleosols were identified and analyzed from all three sites and grouped into five pedotypes ranging in pedogenic maturity: Gray and Gray-Red Types are weakly developed, immature soils formed under poor drainage conditions; Red and Purple Types are moderately developed soils formed under alternating wet/dry conditions; Brown Type are well-developed, mature soils formed under well drained conditions.  A morphology index and two geochemical proxies (Nb and Ba/Sr) provide further information on paleoprecipitation, and drainage conditions.  A conceptual model was developed linking the Nb paleoprecipitation proxy and Ba/Sr drainage proxy to determine landscape changes as a result of precipitation/evaporation versus base level.  Potomac Formation Unit I displays varying dry to wet conditions up section from the unit base.  The morphology index and geochemical proxies provide evidence that Unit I was sub-humid with episodes of saturation and overall drier conditions relative to overlying units.  Paleoprecipitation was the main control on the formation of these paleosols.  Units II (lower Albian to lower Cenomanian) and III (lower Cenomanian) have similar wet and dry conditions upsection through both units.  Paleoprecipitation played a role lower in Unit II although upsection base level exerts more influence on landscape conditions.  The morphology index and geochemical proxies provide evidence Units II and III were deposited under wetter conditions, experiencing sub-humid to humid conditions, with episodes of drying.   Palynology also provides a correlation tool, and was analyzed here to try and establish a higher resolution of correlation. This was attempted using angiosperm diversity patterns, specifically Monocots-Magnoliids, Eudicots and the ratio of Eudicots to Monocots-Magnoliids.  The inconsistent sample material as well as sparse angiosperm populations did not allow for a higher resolved correlation.     A sequence stratigraphic framework was developed for the Potomac Formation.  The Potomac Formation units were subdivided into packages known as Fluvial Aggradation Cycles (FACs).  An analysis of FAC stacking patterns reveals potential sequence boundaries and systems tracts.  FACs support the identification of unit boundaries as sequence boundaries.  FACs also indicate tentative higher order sequence boundaries and provide potential additional correlative surfaces among Potomac Formation sites.   This study reconstructed the landscape showing the variability in climate (precipitation/evaporation) and base-level through time that had a significant influence the formation of coastal plain paleosols.  This enhances the overall understanding of how coastal plain landscapes evolve in transitions towards greenhouse climates during overall transgressions.  The use of FACs has provided a potentially novel method to correlate sites at a higher resolution, creating tie points within these lithologic units.  It also provided further information on the landscape evolution through this time interval, offering information on base-level and accommodation.