INVERTED FLUVIAL TOPOGRAPHY CONDITIONED BY A LATE TRIASSIC PALEO-CHANNEL COMPLEX, NESHAMINY CREEK, PENNSYLVANIA

Topographic inversion occurs during landscape evolution when relatively low-lying topographic elements appear at higher elevations due to resistance to erosion. This study investigated the spatial distribution of lithostratigraphic elements of the Stockton Formation (Upper Triassic) within the Newark Basin. At Newtown (Bucks County, Pennsylvania) this formation exhibits basal fluvial conglomerates that fine upward into arkosic channel-fill sandstones interbedded with riparian mudstones and incipient paleosols. At two study sites incised by Neshaminy Creek, the contact between the paleo-channel sandstone and floodplain shale was mapped by elevation and across the stream-bank trend. At the northernmost site in central Tyler State Park, the contact occurs at an elevation of ~38 m. Downstream, this contact descends to ~27 m, with red shale also outcropping adjacent to and overlying the distinct lenses of arkosic sandstone. The uppermost shale units are stabilized by pedogenesis and vegetation. Near a small dam, an ~3-m-high promontory of Stockton Fm has been exposed by preferential scour erosion, resulting in an overhang of conglomerate containing rip-up clasts of subjacent riparian deposits that also laterally envelop the channel complex. At stream level, this weathered shale is eroded into niches beneath low-elevation overhangs of sandstone (1-4 m). The asymmetry in stream profile can be explained by incision along a paleo-channel belt and lateral erosion in the direction of the coeval floodplain sequences. At several additional downstream sites, high sandstone ledges follow the trend of paleo-channel network, with narrow riffles and sandstone boulders indicating stream segments where this resistant lithology has been breached, incised, or fully undercut. Our findings demonstrate a substantial control exerted by an antecedent semi-arid Mesozoic fluvial-riparian complex on the position and morphosedimentary processes of a Quaternary stream, with an ongoing trend of accentuating the topographic inversion.