| Summary |
Outcropping Mesozoic and Tertiary strata form extensive hardbottom exposures on the Atlantic continental shelf of Onslow Bay, an open marine embayment off southeastern North Carolina. Modern bottom sediments show textural and compositional similarities to outcropping strata, suggesting that erosion of exposed hardbottom lithologies supplies continental-shelf sediment. Colonization by benthic infauna is the principal mechanism for erosion of exposed hardbottoms, which may be contributing "new" sediment to the continental shelf surface regime. Description and quantification of the mechanisms and resulting modification of hardbottoms by benthic infaunal organisms is crucial to understanding the morphology and sedimentology of the modern continental shelf system. Bioerosion of partially-indurated, Miocene, mudstone lithologies occurs primarily through boring by the pholad clam, Jouanmtia quillingi, and the mud shrimp, Upogebia affinis. Erosion of mudstone surfaces follows a general evolutionary sequence; 1 ) flat, freshly exposed surfaces are created by block breaks and slumps. 2) These surfaces are colonized by benthic infauna that bore into the substrate. 3) With multiple episodes of colonization, mudstone surfaces break down. Because of increasing proximity, the numerous bore holes weaken the substrate, and eventually a surface with intermediate microtopographic relief (3 to 5 cm) develops. 4) Continued colonization of the surface eventually leads to convoluted, high-relief, mature surfaces (>6 cm). Both Jouanmtia and Upogebia bore mechanically. Juvenile pholad clams burrow 3 to 5 cm into the substrate. At maximum depth of penetration, they transform into the non-boring adult stage. The shrimp excavates sediment by scraping with the front appendages to produce an extensive, multi-chambered burrow system usually extending 25 cm or more into the mudstone. Mudstone bioerosion produces considerable quantities of sediment Sediment trap collections indicate that 5.5 kg of sediment/m²/yr (15.0 g/m²/day) erode from erosionally mature mudstone scarps This corresponds to 2100 cm³/m² of mudstone sediment each year, representing a scarp recession rate of 1 ft/14.5 yrs, which agrees with diver estimated scarp recession rates of about a foot per decade. Most of the sediment is mud, and is quickly lost from the system Quartz sand similar to that occurring in fine sand aprons at the base of vertical scarps and covering portions of megarippled gravels on the lower, fiat, hardbottom, constitutes approximately 25% (1.4 kg/m²/yr) of eroded mudstone sediment. This may explain the persistence of fine sand body features. Sediment trap collections also demonstrate that erosion rates vary seasonally. Sediment traps deployed for 14 and 90 days collected an average of 35.6 g/m²/day during the summer of 1994, as compared to 15.0 g/m²/day during a year long deployment from September 29, 1994 to September 14, 1995. The difference between the two rates likely relates to an increase in boring activity during the summer months. Bioerosion, in combination with storm energy, controls large scale changes in hardbottom geomorphology. Rapid recession of the mudstone creates large overhangs of more-slowly eroding Pleistocene limestone (up to 20 m). These overhangs break off during storm events and fall to the base of the scarp, forming rubble blocks. These rubble blocks collectively form extensive rubble ramps. |
