This may be of interest to some. It is a detailed description of the different formations of the Mako Trough.

Book Title: M 45: The Pannonian Basin: A Study in Basin Evolution

Article/Chapter: Preliminary Sedimentological Investigation of a Neogene Depression in the Great Hungarian Plain: Chapter 10

Subject Group: Geologic History and Areal Geology

Spec. Pub. Type: Memoir

Pub. Year: 1988

Author(s): I. Berczi

Abstract:

The Mako-Hodmezovasarhely trench, situated in the southeastern part of Hungary, is filled with more than 7 km of thick Miocene to Holocene sedimentary rocks. Seismic profiles, stratification features in core samples, grain size distributions, and well logs in three boreholes clearly reveal a shoaling-upward sequence within the trough. We have distinguished five principal depositional environments that we interpret (from bottom to top) as: coarse-grained basal turbidites; deep basin fine-grained sediments; delta front turbidites; delta slope sand, silt, and marl; and shallow lake to braided stream deposits.

Text:

INTRODUCTION

The Mako-Hodmezovasarhely trough contains more than 7 km of Neogene-Quaternary sedimentary rocks, and the deepest borehole in Hungary (Hod-I) terminated in Badenian strata in this trough at a depth of 5842 m below sea level. This preliminary study of the basin-fill facies present in this trough is based on a detailed investigation of cores from three boreholes that yield a rough cross-section through the trough: Maroslele-1, Hodmezovasarhely-I, and Bekessamson-1. Analysis of stratification features determined from grain size distribution and well log data enabled us to distinguish five main depositional facies that clearly represent a shoaling-upward sequence in a basin that at times contained steep prograding slopes. We have tentatively identified these five facies as: (1) basal facies; (2) deep basin facies; (3) prodelta facies; (4) delta front-delta slope facies; and (5) delta plain facies.

THE BASAL FACIES

The basal facies is composed of sandy to marly conglomerate and is found only in the axial part of the trench. Borehole Hod-I penetrated 373 m into this unit, but failed to reach even the stratigraphic base of the unit. The age of the upper part of this unit is early to middle Badenian. The interbedded sandstone and marl beds may dip about 7-11° due to deformation and slumping, and the sandstone layers display a coarsening-upward sequence. Most of the sandstone layers show graded bedding (A, B, and C parts of the Bouma sequence), are frequently distorted, and contain marl rip-up clasts. In the nongraded sandstone layers, thin amalgamated beds with small-scale cross-strata or parallel to wavy lamination with marl interbeds are common. This unit can be clearly distinguished from the basal conglomerate that occurs around the margins of the basin. This conglomerate has an obvious psefitic nature, with undistributed bedding and, in contrast to the conglomerate in the deep axial part of the trough, shows little evidence for significant sedimentary transport.

A second influx of muddy conglomerates represented by the Dorozsma Formation appeared in the Pannonian (s. str.) overlying the deep basin facies .

DEEP BASIN FACIES

The deep basin facies overlaps the basal facies in the axial part of the trench. This 900-m-thick series consists of calcareous and argillaceous silty marls of laminated and (to a lesser extent) massive appearance. No bioturbation was observed. This unit is of middle to late Badenian to Pannonian (s. str.) age.

PRODELTA FACIES

The prodelta facies can be divided into two parts: the upper part is more sandy and is considered to be a product of fan deposits (proximal turbidities) represented by thick graded sandstones (A-B, B-C, and C-E parts of the Bouma sequence) and slumped deposits. Marl rip-up clasts are rarer than in the delta front-delta slope facies. Dish structures as well as load structures at sand-marl contacts are common.

The lower part (distal turbidities) are characterized by gradual decreases in the sand bed thickness, in the sand to marl ratio, and in grain size, as well as by the predominance of C-E units of the Bouma sequence. No bioturbation was observed in any part of the prodelta facies.

DELTA FRONT-DELTA SLOPE FACIES

In the delta front-delta slope facies, sandstone and marl may occur as horizontal parallel beds, but the most characteristic feature is the 4-25° dip of the bedding (the most common dip is 5-7°). Similar values can be measured as dips of foresets in regional profiles. The sandstone beds contain load casts, flame structures, and marl rip-up clasts showing well-defined graded bedding, with a predominance of A, A-B, and A-B-C units of the Bouma sequence. In the most steeply dipping part of the sequence, soft sediment deformation (synsedimentary faulting, slumps, rotation of beds, and overturned strata) are common. Bioturbation is abundant in the upper two thirds of this sequence.

DELTA PLAIN FACIES

The delta plain facies is composed of alternating layers of horizontally bedded sandstone, siltstone, and marl. The sandstone and siltstone layers are graded and repeated. Occurrence of small-scale cross-strata as well as bioturbation is also common. Oxidized mudstone intercalations are common, indicating that this region was sometimes dry land. The increasing importance of terrestrial conditions is also indicated by the common occurrence of lignite beds (Reineck-Singh, 1980).




DISCUSSION

Two- and three-dimensional views of the facies discussed above are summarized. We propose that a highly constructional, fluvial-dominated delta system (Reading, 1978) existed in the Mako trench, and that it prograded from northwest to southeast. Such progradation is also indicated by the mineralogical composition of the sandstones: most of them are lithoarenites containing rock fragments and heavy minerals of metamorphic (Alpine) origin. No volcanic debris from the north and east could be detected (Berczi, 1970, 1972).

The Neogene evolution of the trench can be tentatively summarized as follows. At the beginning of Badenian time, the rapidly subsiding areas quickly reached the stage of deep basin (pelitic) sedimentation. The steep slopes of the trench initiated the reworking of coarse-grained sediments as turbidites. Thus, the matrix of the basal facies is suspended mud and chemically precipitated CaCO3, to which were added periodic influxes of coarse-grained sediments. The turbidities may have flowed either along the axis of the trench or down its flanks. Periodic occurrences of steeply dipping marl and sandstone interbeds suggest irregular bottom topography with slumping strata and/or infill of previously formed turbidite channels. (We use "turbidite" to mean predominantly sediment gravity flow deposits.)

The deep basin facies is composed of mud deposited from suspension and chemically precipitated CaCO3. Fossils with CaCO3 shells are encountered very scarcely, thus biogenic effects in CaCO3 precipitation can certainly be excluded. The horizontal bedding shows no disturbances of any kind.

The proximal part of the overlying prodelta facies was dominated by periodic influxes of sand by turbidity flows that exhibit decreasing energy toward the center of the trough(?). The amalgamated sandstone beds (these represent the top of the prodelta series) sometimes adhere to the lower part of the slumps of fans deposited in the surrounding (sublake) channel system. The structure of the sandstone is dominated by the upper units (C-E) of the Bouma sequence and by horizontal bedding.