Gypsum-Karst Collapse in the Black Hills, South Dakota-Wyoming, USA

Povzetek

Medplastno raztapljanje sadre in anhidrita v štirih stratigrafskih enotah starosti od karbona do jure na področju Black Hills, South Dakota in Wyoming, je povzročilo mnogo udorov, ki so se prvotno razvili v zgornjih netopnih kamninah. Udori so prizadeli več naseljenih območij. Podzemeljsko medplastno raztapljanje anhidrita Minnelusa formacije je povzročilo regionalno podorno brečo, velike prekinitve plastnatosti, mnogo vrtač ter brečaste kope, izmed katerih se nekatere razprostirajo 300 m v zgoraj ležeče plasti. Recentni udori so povzročili strme vrtače globje od 20 m, udore v vodnjakih in izvirih, ki so posledica prekinitve sedimentov in onesnaženja ter sveže morfološke stopnje, ki obkrožajo plitve depresije. Veliko plasti sadre je deformiranih zaradi ekspanzije, kar je posledica njene hidracije iz anhidrita. Mnogo sadrinih žilic najdemo vzdolž naključnih razpok v matičnih plasteh sadre. Ob izvirih se pojavlja več vrtač. Ker čelno raztapljanje anhidrita v podzemlju Minnelusa formacije napreduje vzdolž vpada in radialno izven centra dviganja področja Black Hills, bodo ti izviri presahnili, pojavili pa se bodo novi skladni z geomorfološkim razvojem področja Black Hills. Stare brečaste kope, ki so ohranjene v prečnih profilih sten soteske, kaæejo na nekdanji položaj čelnega raztapljanja. Mirror Lake, ki se razprostira proti SZ vzdolž smeri vpada, je lokalna primerjava napredujočega čela raztapljanja.

Intrastratal dissolution of gypsum and anhydrite in four stratigraphic units of Pennsylvanian to Jurassic age in the Black Hills of South Dakota and Wyoming has resulted in many collapse features that have developed primarily in the non-soluble overlying rocks. Subsidence has affected several areas that are undergoing urban development. Subsurface intrastratal dissolution of anhydrite in the Minnelusa Formation has produced a regional collapse breccia, extensive disruption of bedding, many dolines, and breccia pipes and pinnacles, some of which extend upwards more than 300 m into overlying strata. Recent collapse is evidenced by steepwalled dolines more than 20 m deep, collapse in water wells and natural springs resulting in sediment disruption and contamination, and fresh circular scarps surrounding shallow depressions. Many beds of gypsum are contorted because of expansion due to its hydration from anhydrite, and many gypsum veinlets extend downward along random fractures from parent gypsum beds. Several dolines are sites of resurgent springs. As the anhydrite dissolution front in the subsurface Minnelusa moves downdip and radially away from the center of the Black Hills uplift, these resurgent springs will dry up and new ones will form as the geomorphology of the Black Hills evolves. Old dolines and breccia pipes, preserved in cross section on canyon walls, attest to the former position of the dissolution front. Mirror Lake, which is expanding northwestward in a downdip direction, is a local analog of a migrating dissolution front.