The regressive Early-Mid Cambrian ‘Hawke Bay Event’ in Baltoscandia: Epeirogenic uplift in concert with eustasy

摘要: Abstract The lower Middle Cambrian of Scandinavia is reviewed and subjected to sequence stratigraphical analysis. Sequences are defined as transgressive–regressive depositional units bounded by maximum regressive surfaces their correlative conformities. forms part a 2nd order sequence, labelled Supersequence 3 (CS-3). It at the base regional unconformity ascribed so-called Hawke Bay Event, focus this paper. CS-3 comprises least four 3rd sequences, but upper above Triplagnostus gibbus Zone has not been analysed yet supersequence boundary remains undefined for time being. oldest seemingly Early age. There numerous T/R cycles in Acadoparadoxides oelandicus Superzone, here ranked subsequences (4th sequences), classification some these possible, thereby increasing number prospective sequences CS-3. high frequency sea-level fluctuations reminiscent glacioeustasy early Mid possibly represents c. 5 Ma long glacial interval with overall low global frequent changes. identified Scandinavia, primarily reflects an extensive long-lasting epeirogenic uplift western Baltica, using term event misnomer. informally referred uplift. However, initial rapid latest coincided ≥ 100 m eustatic lowering sea-level, leading simultaneous regression East Baltic area elsewhere. This forced assumed representing Event s.str. In Estonia–Latvia–Lithuania, separating Kibartian Deimenian stages signals regression. gradually differentially subsided during Cambrian, starting Oland–Gotland area, collapse then slowly progressed westwards southwestwards. Subsidence also advanced from Norway eastwards whereas margin Baltica apparently remained uplifted throughout late earliest Ordovician. central last traces eventually disappeared Acidusus atavus Chron. As result greater hiatus southernmost where persisted longer (locally up 6 Ma), causing non-deposition associated minor erosion. inferred have triggered stress-induced isostasy linked major plate-tectonic adjustments, onset subduction adjacent Iapetus Ocean or cessation Timanide collision, both. A series palaeogeographical maps constructed countries. Subcrop onlap unconformity, well isopach formations shown. elongate land that straddled axis Scandinavian Peninsula after subsidence new basin configuration was established. had gentle westwardly sloping sea floor delimited broad along formed submarine sill medial onwards. sill, which later Tremadocian, restricted exchange water masses deeper parts epicontinental became characterized dysoxic anoxic conditions. resulting organic-rich deposit Alum Shale. rose eustatically terminal lowstand (in strict sense) reach temporary highstand A. Superchron. fell abruptly again end Superchron Paradoxides paradoxissimus Overall, on scale, result, clastic supply relatively high. principal sediment east; initially amounts clastics were supplied erosion limited, probably due extremely flat topography. influx east resulted progradation, progressively shifting coastline westwards. Eventually it reached position roughly corresponding present-day coast mainland Sweden. addition (s.str,) significant regressions occurred pinus – Pentagnostus praecurrens Chron (here named Faludden Lowstand) T. (the Mid-gibbus Lowstand). Several unnamed lowerings P. rapid, rise beginning Chron, estimated 60–70 m, Oelandicus Drowning. Fast rises start middle Forsemolla Exsulans drownings, respectively. From onwards into there (Atavus Highstand). latter 100 m first higher than just prior transgression shifted 500–700 km onwards, extreme starvation prevailed covering Scandinavia. Simultaneously, mid-shelf net sedimentary bypass zone, either regular outboard transport storms recurrent erosive events lowstands. Two phases recognized, termed Depositional Phases 4. Phase encompasses transient increase lowstand, DP4 covers strongly starved conditions widespread dysoxia sea-level. gradual expansion oxygen minimum zone developed independently rising suggestive stratified column, presumably created silled likely, however, general ‘background’ level oceans low, perhaps sluggish circulation greenhouse world. stages, Bodan Almbackenian, introduced superzones Timofeevia lancare acritarch assemblage located within trilobite Zone. subdivided jemtlandicus subzone Bailiaspis dalmani subzone.