Influence of palaeoclimate and the greenhouse effect on Hettangian clay mineral assemblages (Holy Cross Mts. area, Polish Basin)

Geo log i cal Quar terly, 2009, 53 (3): 363–368

In flu ence of palaeoclimate and the green house ef fect on Hettangian clay min eral as sem blages (Holy Cross Mts. area, Pol ish Ba sin)

Pawe³ BRAÑSKI

Brañski P. (2009) — In flu ence of palaeoclimate and the green house ef fect on Hettangian clay min eral as sem blages (Holy Cross Mts.

area, Pol ish Ba sin). Geol. Quart., 53 (3): 363–368. Warszawa.

Hettangian clay min eral as sem blages from the Holy Cross Mts. mar gin (south east ern part of the epicontinental Pol ish Ba sin) were mostly con trolled by cli ma tic con di tions and weath er ing re gime. Hettangian claystones and mudstones were de pos ited in con ti nen tal and ma - rine-mar gin palaeoenvironments in a warm cli mate, mostly with year-round hu mid ity. The pro nounced, long-term green house con di - tions in ten si fied chem i cal weath er ing in the hin ter land. Re work ing and redeposition of an cient sed i ments caused by tec ton ics and/or by sea-level changes and early diagenesis may have mod i fied the clay min eral con tent in the ear li est Hettangian. Burial diagenesis and telodiagenesis changed the clay min eral com po si tion only lo cally.

Pawe³ Brañski, Pol ish Geo log i cal In sti tute-Na tional Re search In sti tute, Rakowiecka 4, PL-00-975 Warszawa, Po land; e-mail:

pawel.branski@pgi.gov.pl (re ceived: July 01, 2008; ac cepted: May 07, 2009).

Key words: Hettangian, Holy Cross Mts. mar gin, clay min eral as sem blages, palaeoclimate, green house ef fect.

INTRODUCTION

A wide range of geo chem i cal, sedimentological, lithological and palaeontological ev i dence sug gests that the in - ter val at the end of the Tri as sic and the be gin ning of the Ju ras - sic was a time of ma jor en vi ron men tal changes. In the Late Tri - as sic the supercontinent Pangaea be gan to frag ment. Rift ing events were ac com pa nied by re or ga ni za tion of the sub si dence pat tern and by ba saltic vol ca nism that formed the larg est ig ne - ous prov ince in Phanerozoic time (Marzoli et al., 1999, 2004;

Hames et al., 2000). Car bon-iso tope anom a lies have been re - ported world wide that may in di cate co eval global dis tur bances in biogeochemical cy cles. Car bon di ox ide outgassing and global warm ing, in duced by flood ba salt vol ca nism of the Cen - tral At lan tic Mag matic Prov ince (CAMP), as well as a sea-level lowstand (Hal lam, 1997) might have trig gered the sud den re - lease of meth ane hy drates and a pos i tive feed back mech a nism that in turn caused green house ef fect, cat a strophic cli mate change and pro nounced bi o log i cal turn over at the end of the Tri as sic (Palfy et al., 2000, 2001; Hesselbo et al., 2002, 2007;

Guex et al., 2004). The Tri as sic–Ju ras sic bound ary in ter val is char ac ter ized by ev i dence of global warm ing. Such ev i dence de rives from many palaeobotanical and geo chem i cal stud ies

(e.g., Fowell and Olsen, 1993; McElwain et al., 1999; Palfy et al., 2001; Hesselbo et al., 2002; Jenkyns et al., 2002; Co hen and Coe, 2002, 2007), but on the other hand a “cool phase” co - in ci dent with the T–J tran si tion was pos tu lated by Hub bard and Boul ter (2000). Changes in sea-wa ter Os-iso tope and Sr-iso - tope (Fig. 1) re cords in di cate that ero sion and weath er ing of the CAMP started soon af ter its em place ment (Co hen and Coe, 2002, 2007). At the be gin ning of the Sinemurian much of the ba saltic cover had been re moved by chem i cal weath er ing (Co - hen and Coe, op. cit.). Such ac cel er ated weath er ing and ero sion would not have been pos si ble with out a sub stan tial in crease in rain fall. The arid ity-hu mid ity pat tern is an other con tro ver sial prob lem of Rhaetian–Hettangian time. Gen er ally, the changes in cli mate-sen si tive min er als, sed i ments and palaeosols re flect in creas ing hu mid ity (e.g., Halllam, 1985; Arndorff, 1993;

Ruffell et al., 2002; Ahlberg et al., 2003; MÝrk et al., 2003), but the pat tern of cli mate change was com pli cated and may not have been ex pressed glob ally.

RESULTS

In the area stud ied (Fig. 2) clay de pos its are par tic u larly com mon in three for ma tions of the Lower Ju ras sic: the Zagaje

SHORT COMMUNICATION

Fig. 1. Vari a tions in the Sr-iso tope com po si tion of sea wa ter, ex pressed as the ⁸⁷Sr/⁸⁶Sr ra tio, from the lat est Tri as sic to the Mid-Ju ras sic (af ter Co hen and Coe, 2007, sim pli fied by the au thor) Note the sud den in creases of ⁸⁷Sr/⁸⁶Sr ra tio in the lat est Rhaetian and early Toarcian in re sponse to the in ten si fi ca tion in con ti nen tal weath er ing fol lowed global warm ing. Black line in di cate the lat est Rhaetian–Hettangian time in ter val

Fig. 2A — stud ied area and the ex tent of the Hettangian ba sin; B — lo ca tion of bore holes and ex po sure (men tioned in text and shown on Fig ure 5) on a back ground of the sim pli fied

geo log i cal map of the Holy Cross Mts. re gion

Db — D¹brówki, Gr — Gródek, MK — Mroczków–Kraszków, Mr — Mirzec, Pd — Podole, S³ — So³tyków (Odrow¹¿), SP — Stare Pole

Fm., the Przysucha Ore-bear ing Fm. and the Ciechocinek Fm., of Toarcian age (Ta ble 1). In the Hettangian suc ces sions, clay min eral as sem blages com prise pre dom i nantly de tri tal kaolinite and illite with sub or di nate chlorite and only trace amounts of smectite, but dif fer ences in the quan ti ta tive min eral com po - si tion are com mon. In many Hettangian ho ri zons/strata very high kaolinite con tent was ob served. The gen eral re sults of the stud ies in 1968–2006 are given in Ta ble 2 and il lus trated in Fig ures 3 and 4.

The clay min eral as sem blages of the al lu vial-lac us trine Zagaje Fm. (ear li est Hettangian) dis play sig nif i cant vari a tions in the kaolinite/illite (K/I) and Al

O

/K

O ra tio. In par tic u lar, the basal al lu vial clay de pos its are char ac ter ized by a pre dom i - nance of kaolinite or illite that oc curs in vari able pro por tions. In many cases the orig i nal clay min eral as sem blage of Zagaje Fm.

was mildly to strongly trans formed by early diagenetic pro - cesses in swampy en vi ron ments, where sig nif i cant hydro - chemical and Eh–pH changes took place. Kaolinite is in gen -

eral the most abun dant clay min eral in the Przysucha Ore-bear - ing For ma tion (mid dle-late Hettangian). Clay min er als in the re stricted ma rine (la goonal) claystones and mudstones of Przysucha Ore-bear ing Fm. show some quan ti ta tive vari abil ity.

De po si tion in the quiet brack ish-ma rine en vi ron ments pos si bly pro tected the de tri tal clay min er als from pedogenic and early diagenetic over print.

Influence of palaeoclimate and the greenhouse effect on Hettangian clay mineral assemblages 365

Stage Lithoformation Clay de pos its con tent [%]

Toarcian

hiatus

Borucice Fm. 5

Ciechocinek Fm. 25

Pliensbachian

Drzewica Fm. 5

Gielniw Fm. 10

Sinemurian Ostrowiec Fm. 5

T a b l e 1 Lithostratigraphical sub di vi sion of the Lower Ju ras sic and the clay de pos its con tent on the Holy Cross Mts. mar gin

(based on Pieñkowski, 2004 and Kozydra, 1968)

For ma tion Age En vi ron ment MI/Q K** I(Ch,I–S)** K/

I(Ch,I–S) Al2O3 K2O Al2O3/

K2O Clay min er als

as sem blage Source of data

PRZYSUCHA ORE-BEARING FM.

mid–

late Hettangian

brack ish- ma rine (la goon,

delta, embayment)

1.3 58 42 1.4 19.70 2.05 9.6 K>I>Ch (I-S) Brañski

(2007)*

1.4 60 40 1.5 25.22 2.23 11.3 K>I>Ch Brañski (1988*,

1990*, 1993*)

– 61 39 1.6 – – – K>I>Ch Pieñkowski

(1981)*

7.3 57 43 1.3 29.85 2.45 12.2 K>I Maliszewska

(1968)

3.6 53 47 1.1 27.21 2.77 9.8 K?I Kozydra

(1968)

ZAGAJE FM. ear li est Hettangian

con ti nen tal (floodplain.

lake, swamp)

1.6 58 42 1.4 26.86 2.41 12.0 K>I>Ch (I–S) Brañski (1988*, 1993*)

– 53 47 1.1 – – – K>I>Ch (I–S) Pieñkowski

(1981)*

3.0 48 52 0.9 24.85 2.55 9.8 I?K Maliszewska

(1968)

2.7 49 51 1.0 24.23 2.81 8.6 I = K Kozydra

(1968)

Ch — chlorite, I — illite, I–S — illite/smectite mixed-lay ers, K — kaolinite, MI — clay min er als, Q — quartz (and the rest of min er als); * — un pub lished;

** — SMI = 100%

T a b l e 2 The main re sults of the clay min eral com po si tion and Al-, K-ox ides con tent in the Hettangian clay de pos its

(com piled by the au thor on the ba sis of anal y ses from 1968–2006)

Fig. 3. Kaolinite vs. illite con tent in the claystones and mudstones of se - lected Lower Ju ras sic for ma tions from the Holy Cross Mts. mar gin (on the ba sis of data from Kozydra, 1968 and Maliszewska, 1968) Note the dif fer ence be tween Hettangian (Zagaje Fm., Przysucha Ore-bear - ing Fm.) and Lower Toarcian (Ciechocinek Fm.) clay min eral con tent

INTERPRETATION AND DISCUSSION

De tri tal clay min er als are key to un der stand ing past changes in weath er ing re gime, be cause they rep re sent the fi nal prod uct of the con ti nen tal weath er ing pro cess. Cli mate changes may con trol the clay min eral com po si tion di rectly via the tem - per a ture and pre cip i ta tion con di tions on the con ti nent, or in di - rectly via feed back mech a nisms in volv ing sea-level changes, arid ep i sodes and sed i ment re work ing (e.g., Singer, 1984;

Chamley, 1989; Thiry, 2000; Cedric et al., 2006). Clay min er - als in the Hettangian claystones and mudstones of the Holy Cross Mts. area are largely de tri tal and show a gen er ally weak diagenetic over print due to mod er ate burial and to the fairly closed hydrological sys tem (Brañski, 2007). Telodiagenetic trans for ma tion de vel oped only lo cally af ter the Late Cre ta - ceous/Paleogene in ver sion of the Mid-Pol ish Trough (cf.

Kozydra, 1968).

In the Hettangian, the area stud ied was lo cated at ap prox i - mately 45° palaeolatitude in a wide zone of warm-tem per ate cli mate (Chan dler et al., 1992; Sellwood and Valdes, 2006).

The typ i cal Hettangian clay min eral as sem blage cor re sponds to a warm-tem per ate cli mate with year-round hu mid ity (cf.

Singer, 1984; Chamley, 1989; Ahlberg et al., 2003) char ac ter - is tic for mid lat i tudes. How ever, the abun dance of de tri tal kaolinite in many Hettangian beds is con sis tent with the con - cept that the sed i ments were de rived from the ero sion of a

“trop i cally” weath ered cover (e.g., Wil son, 1972; Chamley, 1989; Arndorff, 1993). The ex ten sive flu vial-lac us trine and

delta-la goonal sys tems acted as traps for the kaolinite formed on the hin ter land. Most prob a bly, the kaolinisation was fur ther in ten si fied by ex change of CO

with the Hettangian green - house at mo sphere. Re cent stud ies in di cate a sud den in ten si fi ca - tion in con ti nen tal weath er ing fol low ing a “green house” ef fect at the Tri as sic–Ju ras sic bound ary (Co hen and Coe, 2007).

There is much sedimentological ev i dence (flood ing events, storm de pos its) for heavy rain fall and vi o lent weather con di - tions also in the Pol ish Ba sin (Pieñkowski, 1981, 2004). Such phe nom ena are typ i cal of a green house ef fect. Gen er ally, kaolinite is the most abun dant clay min eral in the Przysucha Ore-bear ing For ma tion (mid–late Hettangian). The tec tonic qui es cence and rel a tively low re lief of the ter rain, com bined with “green house” con di tions (high tem per a tures and in tense rain fall), would have greatly fa voured chem i cal weath er ing rather than me chan i cal ero sion. The kaolinite to illite ra tio in the Przysucha Ore-bear ing For ma tion ap pears to vary geo - graph i cally and ver ti cally (Fig. 5). The lat eral changes (Fig. 5A) re flect the prov e nance and min er al og i cal di ver sity of par ent rocks and soils in the source ar eas. A de crease in kaolinite con tent at the top of the Hettangian (Fig. 5B), may have been caused ei ther by a tem per a ture de crease and/or rain - fall re duc tion, which fol lowed wan ing of a green house ef fect and tran sient sea-level fall in the lat est Hettangian.

The re la tions be tween clay min er als in the Zagaje Fm. are more com pli cated, par tic u larly in the lower part of the suc ces - sion. The in flu ence of the source area rocks can be ob served in the dif fer ent char ac ter of the basal clay as sem blages, re flect ing the deg ra da tion of Norian or Rhaetian de pos its. These sed i -

Fig. 4. Av er age gen eral min eral com po si tion (A) and clay min eral com po si tion (B) in the claystones and mudstones from Hettangian for ma tions (on the ba sis of data from Pieñkowski, 1981, 2004 and un pub lished stud ies of the au thor)

POF — Przysucha Ore-bear ing Fm., SF — Sk³oby Fm., ZF — Zagaje Fm.

ments were re de pos ited in var i ous amounts to pro duce illite or kaolinite spikes in basal beds of the Hettangian suc ces sion.

Changes in the ero sion rate and in con se quence in the illite or kaolinite con tent in the the basal beds of Hettangian might have been in flu enced by a sea-level lowstand (Pieñkowski, 2004) and tec tonic re ac ti va tion (Brañski, 2006) cou pled with pos si ble arid ep i sodes and sub se quent ero sion of older Norian or Rhaetian de pos its. Usu ally, the lithological, sedimentological and min er al og i cal re cord of the lower Hettangian cor re sponds with gen er ally warm and hu mid con di tions (Gierliñski et al., 2003; Pieñkowski, 2004; Brañski, 2007). The very mi nor con - tent of smectite in the suc ces sions in di cate that there were no se vere sea sonal arid ity phases. How ever, one should bear in mind that the acidic con di tions that char ac ter ized swampy en - vi ron ments in the ear li est Hettangian, would lead to early diagenetic trans for ma tion of Al-smectite into kaolinite (cf.

Saez et al., 2003). Some fos sils of xeromorphic plants

(Hirmeriella muensteri) and the pres ence of com mon char coal in the So³tyków ex po sure (Reymanówna, 1991;

Wcis³o-Luraniec, 1991; Ziaja, 1991; Gierliñski et al., 2003) sug gest the con tin u a tion of ep i sodic and/or sea sonal dry ness, de spite the stepwise rise of hu mid ity in the tran si tional palaeoclimatic phase.

CONCLUSIONS

The Hettangian clay min eral com po si tion from the Holy Cross Mts. mar gin was mostly con trolled by green house cli ma - tic con di tions and in tense chem i cal weathering.

Re work ing of the an cient sed i ments and the early diagenesis could mod ify sig nif i cantly the clay min eral content.

The burial diagenesis and telodiagenesis changed the clay min er als com po si tion only in a lo cal scale.

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