キーワード：同時異相，扇状地状三角州，馬上礫岩層，白沙層，秩父盆地新第三系．

---

---

Tertiary System in the Northwest Corner of the Chichibu Basin, Saitama Prefecture
−with the Contemporaneous Heterotopic Facies Between the Moe Conglomerate and Shirasu Formation −

Yukio NAGANUMA
Omiya Municipal Nature Study Homes for the Youth, 2847-2 Miyasato, Tateiwa, Minami-Aizu-gun, Fukushima 967-03
and
Saburo KANNO
7-11-1 Shakujii-cho, Nerima-ku, Tokyo 177

Abstract
The Tertiary System in the Chichibu Basin are divided into the Hikokubo, Oganomachi, and Chichibumachi Groups in ascending order.
The Hikokubo Group consists of the elastic sediments showing transgressive facies,e.g., the group is fundamentally subdivided into the Moe Conglomerate, Shirasu Formation and Tomita Mudstone in ascending order. Moreover, the Shirasu Formation is subdivided into three members as the lsamado Sandstone, Shirasuiwa Arkosic Sandstone and Naragawabashi Sandstone in ascending order.
The Moe Conglomerate is generally measured in a several meters thick conglomerate. However, it reaches about 160 meters in thickness in the northwestern corner of the basin. Here, the lowest part of the formation (10 meters in thickness) consists of the angular pebbles of various size containing boulder size derived from the rock of the Chichibu System.
The mid-horizon of the formation consists of conglomerate (150 meters in thickness), which is characterized by huge boulders of granitic rocks as well as rounded pebbles derived from the Chichibu System. Namely, the horizon from 25 to 85 meters above the base includes huge boulders of granitic rocks such as granite, granitic gneiss, augen-gneiss and quartz-porphyry. The ratio of the granitic rocks to other constituent of the conglomerate attains to 52% at the horizon of 45 meters above the base. The drastic change of the constituent ratio of granitic rocks to the other non-granitics among the conglomerate is clearly distinct in the mid-horizon of the Moe. Such drastic change seems to be due to the abrupt change of slope of river flow of the Paleo-Iwadonozawa.
The mid-horizon of the Moe Conglomerate distributed in the northwestern corner of the basin has a relationship of contemporaneous heterotopic facies with the Shirasu Formation distributed in the northern part of the basin. Namely, the horizon from 25 to 85 meters above the base in the Moe is correlative with the Shirasuiwa of the Shirasu. The former represents a fluvial fan-like deltaic deposits and it is inter fingering laterally into the marine deposits of the latter.
The Sirasu Formation distributed in the northern part of the basin shows shallow marine deposits which is transported by the paleocurrent running towards east-north-east from the estuary of the Paleo-Iwadonozawa.