Late Pleistocene lacustrine sediments and their relation to red soils in the Northeastern margin of the Dinaric Karst

Abstract

A large karst doline at section Hrastje – Lešnica in the Dolenjska region (SE Slovenia) was uncovered during the construction of Slovene highway No. A2. Its fill consists of brownish-yellow clay to silt with plant remains and ferrugineous coatings after root casts and gastropods (paleosol horizon) in the bottom, and overlying thick lacustrine laminated grey clayey sediments which were partly rubified. Brownish-yellow clay to silt contains quartz, chlorite, muscovite and feldspars transported as external clastic material from evolved karst and non-carbonate landscapes from surroundings into the site. The material is well weathered only in the area of the paleosol horizon. The strongly impoverished malacocoenosis indicates any Quaternary warm phase characterized by light semi-open forest with patches of open ground habitats. Only the last paleomagnetic sample in the bottom of sediment sequence shows reverse polarity of magnetic field and represents the geomagnetic excursion, i.e., the Blake excursion at ca 120–112 ka (MIS 5e), rather than Brunhes/Matuyama boundary at 0.78 Ma (MIS 19). Thick lacustrine laminated grey clayey sediments above are also dominated by quartz, muscovite, chlorite and feldspar. That overlying sediment was almost unweathered (content of feldspars, muscovite and chlorite); it was only slightly rubified on its surface, in middle part of the section and at the contact with the underlying karstified limestone slope of the depression. The grey sediment has a different mineralogical composition than underlying soils (e.g., lack of quartz, chlorite) and non-carbonate residue of the host limestone. Therefore, the grey sediments could not serve as a parent (source) material for terra rossa formation in the broader area (i.e., polygenetic red soils developed in paleoclimate related to current Mediterranean climatic conditions). Laminated grey sediment was deposited in a rather cold climate. Relatively poor palynospectra may indicate transport of pollen grains out of the depocentre with flowing water and/or the rapid deposition. The latter is supported by insufficiently centered paleosecular variations. Plant assemblages indicate that the dominant cover of the surrounding landscape was temperate climatic zone riparian forest with some quite humid environment as wetlands and ponds on periodically flooded plain. The regional correlation, based especially on an abundance of Fagus, indicates the deposition at the beginning of the last glacial cycle (Würmian) in its warmer substage – MIS 5c (ca 105–95 ka). All paleomagnetic samples from this part of the sediment section show normal magnetization and negligible clockwise rotation of 1.8° ± 4.7°.

Key words: karst sediments, mineralogy, gastropods, palynology, paleomagnetism, paleoenvironment, Dolenjska region, Slovenia.

Pozno pleistocenski jezerski sedimenti in njihova povezava z rdečimi tlemi na severovzhodnem robu Dinarskega krasa

Na Dolenjskem krasu je bila med graditvijo trase A2 slovenskega avtocestnega križa na odseku Hrastje–Lešnica razgaljena večja vrtača. V spodnjem delu je bila povsem zapolnjena z rjavkastorumenim sedimentom glinene do meljaste frakcije, v zgornjem delu pa z debelim zaporedjem laminiranega sivega glinenega sedimenta, ki je bil ponekod rahlo rubificiran. Rjavkasto rumeni glineni do meljasti sediment na dnu vrtače, v katerem so posamezni rastlinski ostanki, s koreninami povezane ferigene skorje in gastropodi (paleotalni horizont), vsebuje kremen, klorit, muskovit in plagioklaze. Ti so bili preneseni v vrtačo kot klastični material z bližnjih območij razvijajočega se kraškega in nekarbonatnega (fluvialnega) površja. Dobro preperel material je le v območju paleotalnega horizonta. Močno osiromašena malakocenoza nakazuje eno od toplih faz kvartarja, ko so prevladovali svetli gozdovi z redkimi drevesi in jasami odprtih talnih habitatov. Samo zadnji od paleomagnetnih vzorcev na dnu raziskanega sedimentnega zaporedja kaže reverzno polarnost magnetnega polja. Menimo, da ta reverzna polarnost kaže na t. i. Blakeovo geomagnetno ekskurzijo pred cca 120.000 in 112.000 leti (MIS 5e) in ne meje Brunhes/Matuyama pred 780.000 leti (MIS 19). Tudi v debelem zaporedju sivih laminiranih glinastih jezerskih sedimentov prevladujejo med minerali kremen, muskovit, klorit in glinenci. Glede na ohranjenost glinenih mineralov, muskovita in klorita sklepamo, da so ti sedimenti skoraj neprepereli; rahlo so rubificirani le na površju, v osrednjem delu profila in na stiku z zakraselo podlago vrtače. Mineraloška sestava sivih sedimentov je drugačna kot v spodaj ležečem talnem horizontu, kjer je manj kremena in klorita, razlikuje pa se tudi od nekarbonatnega preperinskega ostanka apnenčaste prikamnine. Kot tak torej siv sediment ni izvorni material na sosednjih območjih pogoste terre rosse (poligenetska rdeča tla, razvita v pleoklimatskih razmerah, podobnih današnjemu mediteranskemu podnebju). Laminirani sedimenti so se odložili v razmeroma hladnem obdobju. Dokaj siromašen palinospekter lahko kaže po eni strani, da so bila pelodna zrna odstranjena iz depocentra s tekočo vodo, po drugi pa na visoko hitrost sedimentacije. V prid temu procesu pritrjujejo tudi premalo centrirane paleosekularne variacije. Rastlinske združbe nakazujejo, da je bila bližnja okolica v glavnem porasla z obrežnimi gozdovi zmernega podnebnega pasu, kjer so se v humidnih obdobjih poplavljenih ravnic občasno pojavljala tudi mokrišča in manjše ojezeritve. Regionalne korelacije, ki temeljijo predvsem na veliki količini pelodov bukve (Fagus) kažejo, da so se sivi glinasti sedimenti odlagali v toplejšem obdobju na začetku zadnjega poledenitvenega cikla (Würm) med t. i. MIS 5c pred okoli 105.000 do 95.000 leti. Vsi paleomagnetni vzorci tega dela sedimentnega profila kažejo normalno magnetizacijo in zanemarljivo rotacijo 1,8° ± 4,7° v smeri urinega kazalca.

Ključne besede: kraški sedimenti, mineralogija, polži, palinologija, paleomagnetizem, paleookolje, Dolenjska, Slovenija.