Origin of Nodular Structures in Devonian Limestones of the Cracow Upland, Southern Poland
BEDNARCZYK, JOLANTA B., Southern Methodist University, Dallas, TX
The origin of nodular structures in limestones, whether by mechanical and/or chemical processes, poses serious interpretational problems for sedimentologists. Outcrops of Upper Givetian-Lower Frasnian slope-facies carbonates of the Debnik area, Cracow Upland, have been studied to determine the genesis of their extensive nodular structures. The following types of features have been recognized: (I) nodular and wavy limestones composed of isolated, ellipsoidal, peloidal nodules within marly matrix; (II) flaser-nodular limestones (with solution seams); and (III) conglomeratic nodular limestones composed of irregular micritic fragments plus rounded intraclasts and plasticlasts.
Oxygen isotopic analyses of the nodules fall into three distinct groups. The first ranges from -4.4 to -5.0% O(18) and includes Type I nodules and lithified nodules of Type III; the second grouping ranges from -5.0 to -5.3% and includes some Type II nodules and Type III plasticlasts; the third group, ranging from -6.0 to -7.4%, contains samples from intervening marls and residual seams plus some Type II nodules. The C(13) values for these nodules range from -1.5% to +1.0% (PDB); the Type 1 peloidal (bacterial?) nodules are the most negative.
From these field and laboratory observations, one can conclude that nodular and wavy limestones originated by early diagenetic concretionary cementation and subsequent pressure solution; conglomeratic-type nodular limestones were produced by bioturbation and erosional reworking of partially cemented deposits; flaser limestones formed by pressure solution during burial diagenesis. Thus, the nodules in these rocks were formed over a long time period and by several processes, a conclusion which probably applies equally to many other carbonate units.
AAPG Search and Discovery Article #91004 © 1991 AAPG Annual Convention Dallas, Texas, April 7-10, 1991 (2009)