Asosiasi Fasies dan Lingkungan Pengendapan Batuan Sedimen Pliosen-Pleistosen Lembah Bumiayu, Jawa Tengah

Abstract

Stratigrafi regional pada Lembah Bumiayu memiliki lingkungan pengendapan yang relatif berbeda terdiri dari Formasi Kalibiuk, Formasi Kaliglagah yang terendapkan pada lingkungan laut, Formasi Mengger dan Formasi Gintung yang terendapkan pada lingkungan non laut. Secara stratigrafi daerah penelitian tersusun atas 10 litofasies secara deskriptif, yaitu batulempung masif karbonat (Fm), batupasir sedang laminasi karbonat (Sh), batupasir kasar silang-siur (Sp), batupasir kasar laminasi (Sh), batupasir kasar laminasi karbonat (Sh), batupasir sedang silang-siur karbonat (Sp), Konglomerat massif karbonat (Gcm), Konglomerat massif (Gcm), Batupasir sedang silang-siur (St), dan batupasir sedang laminasi (Sh). Berdasarkan asosiasi fasies dengan pendekatan model fasies, rekonstruksi paleogeografi daerah penelitian terbagi menjadi 5 tahapan event perubahan asosiasi fasies yaitu pada N19 – N20 terendapkan fasies offshore dengan pengendapan progradasi, Pada N20 – N21 mengalami progradasi terendapkan fasies beach kemudian terjadi retrogradasi dan terendapkan fasies shoreface, Pada N21 – N22 terjadi pengendapan progradasi sehingga terendapkan fasies braided channel fluvial system, dan pada N22 – N23 terjadi kembali pengendapan retrogradasi sehingga terendapkan fasies shoreface.

The regional stratigraphy of the Bumiayu Valley, Brebes Regency, Central Java has a relatively different depositional environment consisting of the Kalibiuk Formation, the Kaliglagah Formation which is deposited in the marine environment, the Mengger Formation and the Gintung Formation which are deposited in the non-marine environment. This becomes the basis for knowing the paleogeography of the Bumiayu Valley through facies correlation analysis. This study aims to determine the geological conditions and identify the correlation of facies associations with paleogeographic reconstruction in the study area using geological field mapping methods with data analysis in the form of lithostratigraphy. Based on the facies association, the paleogeographic reconstruction of the research area was divided into 5 stages of the facies association change event, namely at N19 - N20 deposited offshore facies with deposition of progression, N20 - N21 experienced deposited beach facies prograding then retrograding and depositing facies shoreface, At N21 - N22 there was prograding deposition so that the braided channel fluvial system facies were deposited, and at N22 - N23 retrograding deposition occurred so that the shoreface facies were deposited.