Possible Late Pleistocene Uplift Chesapeake Bay Entrance

Abstract

Paleontological and lithological studies of engineering borings and boring logs indicate that a buried, subaerial erosion surface of Pliocene (?)-Pleistocene age cuts across clastic sediments of pre-Yorktownian Miocene age in the subsurface and subbottom of the lower Chesapeake Bay area. When the bore-hole data are coupled with the results of subbottom echo profiling and piledriving records, it is possible to construct accurate cross sections of the buried Miocene-Pleistocene contact. The cross sections show "lows" in the erosion surface that may be correlated with the buried channels of the Pleistocene Elizabeth, James, York, and Susquehanna river valleys. Probable channel depths below mean low water at control points are: 100 feet (Elizabeth River, beneath Tunnel no. 1), 155 feet (James River, at Hampton Roads Tunnel), 120 feet (York River, at Yorktown), 158 feet (Susquehanna River, off Cape Charles City), and 160 feet (Susquehanna River, at Fisherman Island, Cape Charles). The channel depths of what is believed to be the buried Susquehanna River valley are less than expected when placed on a curve showing the expectable gradients of that stream during the time of the most-recent, maximum lowering of sea level (ca. 18,000 years B.P.). The discrepancy suggests uplift of that channel of approximately 170 feet in about the last 18,000 years. Pollen analysis and dating of peats and shells immediately overlying the Miocene-Pleistocene contact indicate that the peats were deposited in brackish-water marshes or on boreal flood plains, probably only slightly above sea level, and that they were subsequently submerged and covered by estuarine sediments. The peats date between 10,340 and 15,280 years B.P., and occur at depths of 82-89 feet below mean low water. Because points for these age-depth values fall well above those used in developing curves of eustatic rise of sea level on relatively stable coasts, it seems possible that the peats may have been uplifted as much as 160 feet in the last 15,000 years. Additional possible evidence of uplift within the last 1,900 years also is suggested by dates on a peat and underlying shell bed cropping out on the seaward side of Hog Island on Virginia's eastern shore. The peat bed, assumed to have formed at about high tide, and the shell bed, deposited below low tide, now crop out some 5 feet above mean low water, and date from 1,170 to 1,900 years B.P., respectively. Rates of uplift suggested by three localities of dated horizons indicate an average value of about 1.05 feet per century for the past 15,000 years, although the rate of uplift varied with time and actually involves a calculation for subsidence between 6,000 and 2,000 B.P. The rate of uplift seems to have approximated the rate of eustatic sea-level rise (about 2.8 feet per century), between 15,000 and about 8,000 B.P. Apparent reversal of crustal uplift between about 6,000 and 2,000 B.P., coupled with continued eustatic rise of sea level, allowed for extensive flooding of the Susquehanna valley lowland and eastern shore of Virginia. Crustal uplift appears to have resumed between 2,000 B.P. and the present. The crustal-movement curve for the period 14,000-3,000 B.P. is remarkably similar to that found by Kaye and Barghoorn (1964) for the Boston area. /// В науке Πалеонтологии и Литологии при изучении данныхֽполученных при бурении земных скважин и связанных с результатом изучения эхаֽ полученного посредством измерительного инструмента эхолота, а такҗе приняв во внимание при бурении темп скорости парового молота, позволяет сконструировать график поперечного сечения, погребенного Миоцвн-Плейстоцена (Miocene-Pleistocene) в поддонном отложении. Плейстоцен физиографии обнаруживает в этом поперечном сечении обратный наклон в погребенной далине реки Сускуиханна (Susquehanna). Это противоречие в наклοне внумаетֽ чтο произомло поднятке русла реки почти что на 170 футов οт мыса Βарлз в Βиргинии (Саре Сharlesֽ Vіґginiа), приблизительнο за последние 18000 лет. Aлиз пыльцы и (carbon-14), взятый для анализа из торфа и раковин непосредственно Покрывающих Миоцен-Плейстоцен (в месте соприкосновения бурава), показывает, что отложение Торфа произошло в затопленной равнине. Эта равнина по всей вероятности только слегка возвышалась над уровнем моря и что эти отложения торфа затем были затоплены и покрыты устьевым донным осадком. Давность торФа классиФицируется в пределах от 10340 и до 15280 лет и появляетса на глубине от 82 до 89 Футов ниже среднего низкого уровня воды. Все выме изложенное поқазывает, что слой торфа по всей вероятности был поднят на высоту 160 футов за последние 15000 лет. Норма поднятия видимо имеет приблизительно норму эвстати- ческого поднятия уровня моря (2.8 фута в столетие), в период между 15000 и 8000 лет тому назад до нашей зры....