Evolution of Mantled Karst Along the Blue Ridge-Great Valley Margin, USA: An Environment of Sediment Accumulation and Possible Preservation

Abstract

The Appalachian landscape provides an excellent opportunity to examine an environment that promotes sediment subsidence, accumulation and possible preservation: mantled karst. Mantled karst exists along the Blue Ridge-Great Valley margin from south-central Pennsylvania to central Virginia. Topographically, the mantled karst contains low-relief irregular topography with abundant surface depressions, some of which contain ponds, and in places lacks continual surface drainage and springs. Sand and gravel quarries along the valley margin expose predominantly stream flow, hyperconcentrated flow, debris flow and hillslope (colluvial) deposits that mantle karstic Paleozoic bedrock and carbonate residuum. Unconformable and sometimes erosive contacts between carbonate residuum and clastic sediments, and soft-sediment deformation features have been observed within exposures suggesting subsidence into accumulation and/or preservation space created by geochemical dissolution and collapse that continues today. The likelihood of long-term survival in the geological record is dependent upon the relationship between the subsiding sediment and geomorphic base level. The Blue Ridge-Great Valley mantled karst likely contains both short-term accumulation space that is closer to geomorphic base level, thus more prone to removal by fluvial processes, and preservation space, which occurs well below geomorphic base level, where sedimentary sequences may be stored for prolonged periods of time. Ultimately, both accumulation and preservation spaces can provide insight into depositional processes, landscapes and possibly paleoenvironmental conditions, but the geologic record becomes more distorted and fragmented further back into deep time and as this landscape evolves into the future. Key words: karst, sedimentary environments, subsidence, preservation potential, landscape evolution. Razvoj pokritega krasa na robu obmocja Blue Ridge – Great Valley, ZDA: okolja akumulacije in ohranjanja sedimentov Pokriti kras Apalacev nudi izjemne možnosti preucevanja odlaganja, akumulacije in ohranjanja sedimentov. Za pokriti kras na robu obmocja Blue Ridge – Great Valley med južno-osrednjo Pensilvanijo in osrednjo Virginijo so znacilna topografsko razgibana nižavja s stevilnimi depresijami, ki so lahko tudi ojezerjene, povrsinski odtok pa je ponekod nepovezan. Kamnolomi peska in grusca vzdolž roba doline razkrivajo razlicne fluvialne in pobocne sedimente, ki pokrivajo zakrasele paleozojske karbonate in njihov reziduum. Nekonformni in erozijski stiki med karbonatnim reziduumom in klasticnimi sedimenti ter deformacijske oblike v mehkih sedimentih kažejo na akumulacijo in ohranjanje sedimentov v kraskih prostorih. Casovna stabilnost sedimentov je odvisna tudi od položaja ugrezanja sedimentov glede na erozijski nivo: prostori akumulacije so blizu erozijskega nivoja in zato podvrženi fluvialnemu odnasanju, prostori ohranjanja pa so pod erozijskim nivojem, zato so tam sedimentna zaporedja ohranjena precej dlje. Preucevanje prostorov akumulacije in prostorov ohranjanja nam omogoca boljse razumevaje razvoja pokrajine in sedimentacijskih procesov v njej. Starejsi zapisi so slabse ohranjeni in bolj razdrobljeni. Kljucne besede: kras, sedimentacijska okolja, ugrezanje, potencial ohranjanja sedimentov, razvoj povrsja